Two other numbers before FIRE
I've played with the free calculator on this site, talked with "experts", hit a bunch of other sites. Two other forecasts I found useful in thinking about FIRE that I haven't seen a mentioned much (if at all) and that the calculators don't really seem set up to handle--
1. What if you just stop contributing to your retirement accounts and lived for x number of years using 100% of your income. i.e. live "high on the hog" as they say down here. Take those expensive trips buy that fancy car outright, etc. The idea is yes, you put in more years working, but you don't draw from reserves. Or you could cut back your workload. Obviously people seem to be doing this but I don't see the outlook supported by the programs online.
2. This is similar, and can be done with lots of repeated simulations on most calculators--how much per month in retirement are you working for? i.e. If I retire today my safe withdrawal is X per month. If I work another year, with the same safety factor, I can withdraw X+Delta. Its more a mental idea that made me feel better about early retirement. If I work another full year is it worth the delta per month?
These two combined have me scratching my head a little and I'm still trying to think my way thru it. A year of idea #1 seems to have little to do with the end result while #2 changes it more than I would have thought. Still trying to figure out the programs (and my) assumptions that cause that difference.