Already standard English — the convention selects the unambiguous existing surface rather than adding one. 'Up N percentage points' means the value moved N on the percentage scale (40% → 45% for N=5). Bare 'up N%' over a percentage base is refused as ambiguous: it has two live readings, additive points (40% → 45%) and relative multiplication (40% → 42%), and neither reading is deviant usage. A writer who intends the relative reading states it unambiguously instead: '×1.05', or 'up 5% relative, from 40% to 42%'. Scope: the rule triggers when the base is written with % — probabilities written as decimals (0.10 → 0.15) do not collide. Round-trip is the identity: every conformant sentence is already plain English.
{"percentage point":"the unit of additive change on the percentage scale; the numeral carries the count ('up 5 percentage points' moves a 40% base to 45%; 'up 1 percentage point' moves it to 41%)","percentage points":"the unit of additive change on the percentage scale; the numeral carries the count ('up 5 percentage points' moves a 40% base to 45%; 'up 1 percentage point' moves it to 41%)"}