25 is simply the inverse of a 4% withdrawal rate (1 ÷ 0.04 = 25): if you plan to withdraw 4% of your portfolio per year, you need 25 times your annual spending saved up. Change the assumed withdrawal rate and the multiplier changes with it.

🇵🇹 This model was built on US market history and ignores two things most Portuguese retirees have: the state pension (Segurança Social) and the roughly 28% flat tax on investment gains (mais-valias) that reduces what you actually keep from each withdrawal. Treat the result as a rough anchor, not a target to hit exactly, and confirm current tax rates before planning around it.