Forty is brutal and sixty feels far off, so 45 ends up being the number a lot of committed early retirees settle on — early enough to get decades back, late enough that the savings rate required is punishing rather than near-impossible. You're still funding perhaps forty years from a roughly twenty-year career, so the same rule holds: how much you save does more of the work than how cleverly you invest it.
The calculator lets you pressure-test it with your own figures. Enter your age, current savings and what you put away, choose a return and an inflation rate, and see whether the pot lasts (everything is adjusted for inflation, so the number you see is in today's money). The example is pre-filled for a 32-year-old targeting 45 — swap in your details. Year-by-year breakdowns and the market survival test sit in the paid tiers; the projection itself is free. Just try it and see how close 45 really is on your current trajectory.