This year's annual allowance (tapered on your income where it must be) plus the unused allowance from each of the last three years — each tapered on its own year's income, which is the part hand-waving estimates skip. The same taper function the Hundred Summers engine uses. Your figures never leave your browser.
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The annual allowance is £60,000 — unless your adjusted income that year exceeded £260,000 (with threshold income above £200,000), in which case it tapered away at £1 for every £2, to a floor of £10,000. Carry-forward brings in the unused part of each of the last three years' allowances — each tapered on its own year's income. High earners with variable income routinely get this wrong in both directions.
Relief on personal contributions is capped at 100% of this year's earnings however much allowance you have (employer contributions aren't); carry-forward requires scheme membership in the source year and is consumed oldest-first; and one flexible withdrawal anywhere triggers the £10,000 MPAA with no carry-forward at all. The taper guide walks the definitions; the full Hundred Summers plan models allowance, taper, carry-forward and MPAA inside your whole projection.
Hundred Summers