The 42p figure, properly explained
A higher-rate taxpayer earning £60,000 pays 40% income tax and 2% Class 1 National Insurance on every pound between £50,270 and £125,140. The combined marginal deduction is 42%. Sacrifice £100 of gross salary and your net pay falls by £58 — HMRC and the Treasury collect £42 less, and £100 lands in your pension.
The LISA works the other way. You pay yourself £58 of net salary, top it up with £42 of after-tax cash to make £100, and the government adds 25% of that £100 — £25. Your pension contribution starts at £100 and your LISA contribution lands at £125. The pension wins on day one if your effective retirement marginal rate is anything below 20%, and the 25% pension commencement lump sum means most of any sensible pot is withdrawn at exactly that rate or lower.
Work the maths over 25 years at 6% real growth and the pension's £100 becomes £429. The LISA's £125 becomes £537 — but only if you can actually contribute £125 of after-tax money in the first place, which our higher-rate worker can't because they only freed up £58. To match the salary-sacrifice take-home cost of £58, the LISA contribution must be £58 + £14.50 bonus = £72.50, growing to £311. The pension wins by £118 per £58 of after-tax cost, before any employer NI rebate.
The number that should be on your fridge isn't 25%. It's 42%, because that's the rate of relief you're choosing not to take if you fund a LISA before maxing salary sacrifice.