When you forecast your spending rate, you naturally take some security margin, for example by retiring when your passive earnings gets 1.5x times larger than yo

When you forecast your spending rate, you naturally take some security margin, for example by retiring when your passive earnings gets 1.5x times larger than your targeted spending. And that security margin (which won't be spend most years) will compound. Sure, you might encounter a black swan event which force you out of retirement only 3 years after you started, and at that point your security margin won't have increased your stash by much, but at the same time your skill won't have deteriorated much in 3 years. It also work even if the black swan even happens 20 years after your retirement. You just need to take a security margin your are comfortable with, and project how much you will have after 3, 10, 25 years to see how much you have for unforeseen events.