The field notes in this series each answered one question: how long could you last, what does the debt really cost, when could you stop, what does compounding do. Useful separately — but real life asks them all at once, in the form of a single practical question: a spare £100 exists this month; where does it go?

Maths, it turns out, has an opinion. Not about your values — about ordering: some destinations strictly beat others while certain conditions hold, and the conditions are checkable in a spreadsheet you mostly already own.

The disclaimers at full volume, because this is the page that sounds most like advice: it isn’t — it’s the standard reasoning framework (you’ll find versions of it everywhere serious money guidance is written), organised so you can check your own position against it. Wrappers, employer schemes and tax details are personal; a professional sees the whole board.

The ladder, and why each rung is where it is

1. The minimum-payments floor. Not a choice — missed priority bills and defaults poison everything downstream.

2. A starter buffer — a month or so of crisis floor. Before any optimising: without a buffer, the first surprise becomes new expensive debt, undoing every rung below.

3. The employer pension match. If contributing another 2% gets your employer’s 2%, that’s an instant, guaranteed 100% return before the compounding even starts — nothing legal beats it. Unclaimed match is a pay cut you volunteered for.

4. Expensive debt. Overpaying a 24% credit card is a guaranteed, tax-free 24% return — the amortisation table shows exactly how brutally it compounds in reverse. Everything above ~8–10% dies here, highest true rate first. (Cheap debt — the mortgage, tax-like student loans — is not this rung; that’s rung seven’s judgement call.)

5. The full runway — three to six months, now affordable because the fires are out.

6. Long-horizon investing — pension beyond the match and/or ISA, in real terms, riding the compounding curve. This is where the £100 usually lands for someone whose rungs 1–5 are solid — and where it should not land for someone whose aren’t.

7. The judgement rung — overpay cheap debt vs invest more vs spend on a life you’re deferring: the model you already built, plus values the sheet can’t hold. Reaching rung seven is the win; everything below it was arithmetic.

£ 1–2 · floor + starter buffer ✓ funded 3 · employer match ✓ claimed 4 · expensive debt ✓ cleared 5 · full runway ✓ 5.1 months 6 · long-horizon investing ← lands here ✓ the coin falls past every satisfied rung and stops at the first open one
Each rung has a checkable condition; the £100 stops at the first one that isn't met. No willpower involved — just a ladder and a checklist.

Build it as a status sheet

The point of putting this in Excel isn’t the flowchart — it’s wiring the conditions to your real numbers. One small tab on the net-worth workbook, one row per rung: the condition as a formula (=runway_months >= target_months, =MAX(debts_apr) < 0.08, match claimed as a yes/no), a status cell, and the glow on the first unmet rung. It’s the checks-row pattern pointed at your life: the sheet tells you where the next £100 goes, and — quarterly, when the balance-sheet snapshot updates — tells you when the answer changes.

Two humane notes to finish. The ladder governs spare money, not all money — a life that’s all rungs and no living fails a test spreadsheets can’t run; budget the fun first, ladder the surplus. And don’t optimise across rungs — the classic mistake is investing (rung 6) while a 22% card burns (rung 4) because investing feels like progress and debt feels like the past. The ladder exists precisely because feelings order these things badly and arithmetic orders them well.

One question — where does the next £100 go? — answered once, wired to your numbers, retired forever. That’s this whole series in a sentence: dread, converted to arithmetic, one model at a time.