... there is no magic formula to compute how much you can safely spend in retirement. *I take the responsibility of managing the family assets as my new job in retirement, where being paid for performance takes on the true meaning of the words!
You're right -- none of the formulas I've seen offer much in the way of precision. They either tell you someone's opinion
of what will happen in the future, or, as with Intercst's work (on which FireCalc was based), the facts of what has happened in the past.
As an early-retired friend repeats to me whenever this subject comes up, the problem is simple once you know how long you will live, and what the market returns and inflation will be during your remaining years!