The Down Payment Dial: How to Split Funding Across Cash, Stocks, and SBLOC

When your $300,000 down payment can come from three different places — existing cash, liquidating taxable stocks, or drawing on an SBLOC — the standard mortgage calculator picks one for you and silently moves on. Usually the one with the cleanest narrative. Cash if you have enough; partial liquidation if you don't.

The actual decision space is a dial. Twelve percent from cash plus 38% from a stock sale plus 50% from an SBLOC produces a financial outcome that's different from any of the three "pure" scenarios. Different in cost. Different in tax. Different in ending net worth ten years out. Different in how exposed you are to a market drawdown.

Stockstead is the only mortgage calculator I know of that exposes that dial. This post is what it's for, what to actually do with it, and the version of the trade-off that I think most HNW buyers under-appreciate.

What the dial actually is

The Custom Mix scenario in the calculator takes three percentage inputs that have to sum to 100%:

• % from Cash — drain from your existing cash & equivalents balance
• % from Stocks — liquidate from your taxable brokerage account, realizing capital gains
• % from SBLOC — borrow against your taxable portfolio without selling

A 100/0/0 mix is a pure-cash down payment (and may return Not Feasible if your cash balance is too low). A 0/100/0 mix is a pure-liquidation. A 0/0/100 is a pure-SBLOC. Every other combination — 50/30/20, 30/0/70, 25/25/50 — is a hybrid that doesn't show up in any other comparison-style tool. Each one produces a different cost-and-wealth curve over the analysis period.

The reason no other calculator surfaces this is that the math gets meaningfully harder once you allow mixing. The cap-gains hit scales with the liquidation slice. The interest carry scales with the SBLOC slice. The opportunity cost scales with whatever isn't the cash slice. Standard tools collapse all three into "down payment = $X" and discard the dial. Stockstead doesn't.

The case for the dial: five mixes against one buyer

Same buyer as the lead SBLOC vs Mortgage vs Cash example: $1.5M home, 20% down, $150K cash on hand, $2M in taxable stocks at 50% basis, 7% expected real return, 10-year hold, NY tax footprint (~28.5% combined LTCG), 8.5% SBLOC rate, 6.5% jumbo.

Five different mixes, same buyer:

Mix (% Cash / % Stocks / % SBLOC)	Cap-gains tax	10-yr cost	Net position at year 10
100% / 0% / 0% (pure cash)	—	—	Not Feasible (only $150K cash)
50% / 50% / 0% (cash + liquidate)	$21,375	$568K	$7.30M
50% / 0% / 50% (cash + SBLOC)	$0	$746K	$7.74M
0% / 50% / 50% (liquidate + SBLOC, no cash drain)	$21,375	$872K	$7.96M
0% / 0% / 100% (pure SBLOC)	$0	$946K	$8.18M

The pattern is mechanical and worth saying out loud:

Each step toward the SBLOC adds cost. More borrowing = more interest carry. The total cost climbs from $568K (full liquidation of the gap) to $946K (no liquidation, all SBLOC) — a $378K cost difference over a decade.

Each step toward the SBLOC also adds wealth. The portfolio that didn't get sold compounds at 7% real for ten years. The cash that didn't get spent stays liquid and earns. By year 10, the pure-SBLOC mix ends with $880K more net worth than the partial-liquidation mix, despite costing $378K more in carry.

Net of net, the SBLOC-heavy mix wins by ~$502K in this scenario. That's the part the standard mortgage calculator can't see: the cost line going up and the wealth line going up faster.

The dial is the tool for picking your point on that curve.

Why I added this dial in the first place

I'll be honest about the origin: I was running these numbers for myself, in a Google Sheet, in late 2024. The first version had only the three pure scenarios — cash, liquidate, SBLOC. After about a week of staring at the outputs, I realized I kept asking the same question: "but what if I did 70% of one and 30% of the other?"

The instinct was right. For my own situation, neither pure liquidation nor pure SBLOC was the optimal answer. The optimal answer depended on:

• How much cash buffer I wanted to keep after closing — too little is fragile, too much is wasted compounding
• Which specific lots of stock I'd want to harvest if I liquidated — high-basis lots first to minimize realized gain
• How much SBLOC carry I was comfortable servicing on top of a new mortgage payment in a hypothetical drawdown year
• What other tax events I had pending in the same year — RSU vests, ISO exercises, anything else that was already pushing my bracket up

None of those answers were "100% of one thing." They were all mixes. The dial existed in my head and in my spreadsheet long before it existed in the calculator. The calculator added it because the calculator was supposed to model the actual decision, and the actual decision is tunable.

How to think about where to set the dial

The mechanical answer — set the dial to wherever ending net wealth is highest — is almost always 0/0/100 (pure SBLOC) for a buyer who can carry the interest, in a positive expected-return regime, with a long horizon. The math just rewards leverage when leverage is cheaper than the un-touched compounding rate.

The realistic answer is more nuanced because the math doesn't account for the risk of three things going wrong:

1. Drawdown coinciding with debt service stress. A 30% market correction reduces your portfolio collateral and may compress your overall cash flow at the same moment your SBLOC interest rate is climbing with SOFR. Pure-SBLOC mixes maximize this exposure. A small cash slice — say 20% from cash — gives you a buffer that absorbs the first leg of any drawdown without needing to either sell or service additional interest.

2. Career risk you didn't price in. If your income depends on a single employer and your portfolio is concentrated in that employer's stock, an SBLOC against that portfolio plus a mortgage is a triple-correlated exposure. A small liquidation slice — say 20% from stocks — can de-risk by diversifying out of the concentrated position you'd otherwise be doubling down on. The realized gain is the cost of that de-risking; in many cases it's worth paying.

3. The lender changing the terms. SBLOC contracts let the lender adjust advance rates and maintenance ratios with limited notice. A pure-SBLOC mix is most exposed to that discretion. Hybrid mixes are partially insulated.

For most HNW buyers I've talked to (and for myself), the realistic dial setting ends up somewhere between 20/30/50 and 30/20/50 — meaningful cash to maintain a buffer, modest liquidation to manage concentration, half on the SBLOC for the tax-deferral and compounding benefits. That's not the absolute wealth-maximizing point on the curve, but it's the wealth-maximizing point after you've priced in the risks the calculator doesn't see.

That's a personal judgment, not a recommendation. Yours will be different.

How the dial interacts with the SBLOC interest and principal strategy

The Custom Mix dial pairs with two other settings in the calculator that change the effective economics of the SBLOC slice:

SBLOC Interest Strategy. Two options: Capitalize (interest accrues into the loan balance, so principal grows) or Pay from Cash Flow (interest is serviced from W-2 income or other liquidity). Capitalizing is mechanically simpler but compounds the loan against you over the analysis period. Paying from cash flow keeps the principal flat and shows up as a recurring cost line item rather than a balance growth.

SBLOC Principal Strategy. Two options: Keep Outstanding (carry the line to the end of the analysis period) or Pay From Stock Sales / Pay From Income (eventually retire the line through a planned amortization). Keeping the line outstanding maximizes the compounding benefit but leaves you exposed to rate moves and lender discretion at the end of the period. Paying it down at, say, year 5 from a planned vesting event gives you a clean exit but limits the SBLOC's tax-deferral value to the years you actually carried it.

The interaction with the dial: an aggressive SBLOC slice (say 70%+) combined with capitalizing interest creates a compounding loan balance that can erode collateral coverage even without a market drawdown. If you're going SBLOC-heavy on the dial, plan to service interest from cash flow rather than capitalizing it.

When the dial doesn't matter

For everything I've said about the value of the dial, there are profiles where the optimal mix collapses to one of the three pure scenarios:

• High-cash, low-portfolio buyer. If your cash account exceeds the down payment, the dial answer is "100% cash" and the rest of this post is irrelevant.
• Very high cost basis, no embedded gain. If liquidating doesn't realize a meaningful gain, the SBLOC's tax-avoidance value collapses and the dial leans toward stocks.
• Conservative-return regime. If you expect 4% real instead of 7%, the SBLOC's borrowing cost exceeds the portfolio's compounding rate and the dial leans hard toward cash + liquidation.
• No appetite for leverage at all. A genuine preference for zero portfolio leverage is a legitimate choice. The dial answer is "0% SBLOC" and you accept the wealth cost in exchange for the simpler balance sheet.

In each of those cases the dial doesn't add value — but you can confirm that by running the calculator and seeing the curves flatten.

Run your own dial

Stockstead's Custom Mix scenario lets you set any combination of cash, liquidation, and SBLOC percentages and shows the full ten-year (or thirty-year) cost and wealth curves alongside the three pure scenarios. It's the parameter that should exist in every HNW mortgage tool and currently doesn't.

Open the calculator →

The dial isn't a gimmick. It's the actual decision. Standard tools that pretend it's a binary "cash or mortgage" question are answering a question for a different buyer.

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Sources

• IRS Publication 936 — Home Mortgage Interest Deduction
• IRS Publication 550 — Investment Income and Expenses
• FINRA — Securities-Backed Lines of Credit (Investor Alert)

Educational, not financial advice. Tyler Singletary founded Stockstead while researching his own first home purchase. He's spent 20+ years in software product roles — currently as Product Specialist for AI/ML Startups at AWS, previously CPO/COO at Tagboard, and an executive at Klout (exited to Lithium in 2014) — and has been compensated in RSUs, ISOs, and post-IPO equity at high-growth tech companies for two decades. He is not a licensed financial advisor or CPA. The dial settings discussed here reflect personal modeling and judgment; talk to a fiduciary advisor and a tax professional before acting on any specific allocation.