If the 4% rule is the famous one, the 3% rule is the one long, early retirements lean on. The logic is simple: withdraw less each year and your money is far more likely to outlast you — which matters a lot more when you're funding 40-50 years rather than the 30 the original study assumed. The cost, of course, is that you need a bigger pot for the same income (or you spend a bit less).
This calculator lets you see the trade-off directly: put in your numbers, then flip between 3%, 3.5% and 4% and watch how the survival odds and the income change. Just try it. For a long retirement, that one percentage point is often the difference between "probably fine" and "safe".