Here is the fate of every useful workbook: it outlives your attention. You change roles, go on leave, or simply forget — and someone else opens the file cold. At that moment your model is exactly as good as a stranger’s ability to run it. Most workbooks fail this test not through bad formulas but through missing context: what is this, what feeds it, what do I press, what must I never touch?

The engineering world calls the fix documentation and makes it sound like homework. In a workbook it’s about an hour of habits — most of which you’ve already built without noticing.

What you already did

Walk the course backwards and the inheritable workbook is mostly assembled: sheets named for their jobs, reading order left to right; settings as named cells with comments in the Name Manager; formulas that read like sentences instead of coordinates; a checks panel that says out loud whether the file is healthy; recipes whose steps are named; inputs fenced from machinery; a refresh-and-read routine instead of ritual surgery. Every one of those was sold as a kindness to future-you. Your successor is future-you with less context — same beneficiary, bigger need.

What’s left is the part no structure can carry: the README sheet.

The README sheet: one tab, five answers

First tab in the file, plain text, five headings:

  1. What this is — two sentences. “Monthly consolidation of depot sales; produces the one-page report on the Dashboard tab. Owner: [you]. Since: 2026.”
  2. What feeds it — every source, by location: the SharePoint folder the exports land in, the Forms list, the flow that files attachments. Data lineage is the first thing an inheritor can’t guess.
  3. How to run it — the monthly routine as numbered steps. “1. Check the flow filed this month’s export. 2. Data → Refresh All. 3. Checks panel must read ALL PASS. 4. Export Dashboard to PDF.” If the steps embarrass you by their number, that’s the automation backlog, documented for free.
  4. What never to touch — the two or three genuinely fragile things, named honestly. Every workbook has them; only the author knows where they are.
  5. Decisions and history — a dated line per change: “2026-07: VAT handling moved into query, per finance.” Ten seconds per entry; it’s the file’s decision log, and the answer to “why is it like this?”, which is the question inheritors ask most and records answer best.
README 1 · what this is, who owns it 2 · what feeds it (paths, flows, forms) 3 · how to run it: refresh → checks → send 4 · what never to touch (and why) 5 · decision log, dated Dashboard Inputs Data Calc the machinery, already self-naming
One tab, five answers — the difference between a workbook someone can run next month and one that retires with you.

The test, and the honest limit

The proof is empirical: the handover rehearsal. Ask a colleague to produce the monthly output using only the file — you sit silent. Every question they ask is a missing line in the README; every hesitation marks a fragile spot deserving a fence or a check. Twenty minutes, and the file’s real documentation debt is a written list.

And the limit, from the graduation lesson: some workbooks fail the rehearsal structurally — too much tacit judgement, too many exceptions. That’s not a documentation problem; it’s the signal the process needs a system, and the README you attempted becomes the specification for it.

An hour of writing, one rehearsal, and your work survives your holiday — which is, in the end, the professional bar: not “it works”, but “it works without me.”