I spent a week building 10 Excel templates that actually calculate — post-mortem (and one is free)

I spent a week building 10 Excel templates that actually calculate things — here's what broke, what worked, and where you can grab one for free

Last week I built a small storefront called SheetVault: Excel templates for budgeting, weddings, crypto portfolios, mortgages, job hunting, savings challenges, freelance invoicing, and splitting bills as a couple. Ten paid templates ($4–8), one free lite version, no email wall on the free one.

This is the honest engineering post-mortem. What broke, what I'd do differently, and — because a budget spreadsheet is only useful if you can actually open it — where to get the free one.

The free one

Free Budget Planner Lite — 3 sheets (Setup, Monthly, Dashboard), real formulas, demo data pre-filled so you can see how it works before deleting the demo rows. Direct download, no signup, no email, no "join 4,000 subscribers first". The source file is also on GitHub: sheetvault-free-templates.

It's the lite version of the $5 full planner, so yes, it's also a funnel. But the funnel works only if the free product is genuinely useful on its own — so it tracks income, fixed/variable expenses, savings rate, and a health indicator. That's a complete monthly budget.

What "actually calculates" means

Most template packs on marketplaces are pretty pictures: hardcoded numbers, static "dashboards" that are just colored cells. Ours use real Excel formulas end to end:

• PMT()-based mortgage amortization with extra-payment scenarios (one-time or monthly) and interest-saved-vs-baseline math
• Proportional bill splitting by income share, with a settlement sheet that outputs a live verdict: "Jordan owes Alex: $598.80"
• Weighted offer comparison: 4 job offers × 5 criteria with customizable weights → automatic WINNER
• Every sheet follows the same rule: yellow cells = your input, everything else is a formula. If a user can accidentally overwrite math, the design failed.

Validation was the hard part. I run every file through a formula-evaluation library headlessly (no Excel on the build machine) and diff computed values against independently calculated expectations: PMT checks, savings-rate checks, split totals. A 52-week challenge tracker must show week N × step, projected total step × 1378 — off-by-one errors in week counts are the classic silent killer.

What broke

1. Data validation lists referencing other sheets must be quoted. 'Settings'!$A$2:$A$9 breaks in some readers if the sheet name contains spaces and you forget the quotes. Grep the raw XML for formula1 entries after every build.
2. Unicode in base64 payloads. The storefront embeds .xlsx files base64-encoded in a Cloudflare Worker. Using JS btoa() on strings with em-dashes throws InvalidCharacterError — you need TextEncoder first. Classic.
3. Preview images and file naming drift. One product page 404'd on its cover because the asset was renamed during a refactor. Now every deploy runs a link/asset check over all product pages.

The storefront stack (for the curious)

• Cloudflare Workers free tier: server-rendered product pages, JSON-LD Product schema, sitemap, /api/order that generates a unique-cent amount per order (e.g. $5.37, $6.84) so incoming USDT transfers can be matched to orders automatically via Tronscan API
• Payments: USDT TRC20 straight to a wallet — non-custodial, no payment processor holding funds, no KYC. Plus a second checkout on sheetvault.sell.app (TRX + ETH USDT, files auto-delivered after payment, 3% fee)
• Third checkout for AI agents: every template is also listed on publish.new with an x402 paywall — if an agent (or you, via a 402-aware client) hits the content endpoint, it can buy and download the file directly in USDC on Base, no account needed
• Traffic: Pinterest pins (queue-based, 3–6/day), Dev.to/Medium posts like this one, GSC + IndexNow for search

No, it hasn't made money yet — it's been live for about a week. The honest metric right now is: does the free planner get downloaded and does anyone find the paid ones. I'll write the follow-up with real numbers either way.

Takeaways if you're building templates yourself

• Ship the free version without an email wall. It converts better into trust (and backlinks) than any lead magnet form.
• Validate formulas headlessly. If you can't compute the expected value by hand, you can't test the template.
• Unique-cent order matching is a poor man's payment webhook — crude, but it works with zero infrastructure and zero fees on TRC20.

Grab the free Budget Planner Lite, and if you hate it, at least it cost you nothing but 8 KB.