FYI, our post-FIRE plans are similar: wait until youngest kid goes to college, and then basically don't come home for a year or so. :-) The analysis that I have found very helpful is the "bucket" method. The problem with the 4% rule is that it assumes that I need to sustain the income from my first few years -- when I am paying for college and traveling a LOT and before any pensions/SS kick in -- for the next 30-40 years. I think Goldielocks laid this out first, but the idea is to divide your post-FIRE life into different time periods based on when your income/expenses change, and then calculate how much money you need to fill each bucket. So for ex. maybe from 50-62, you need $100K/yr; that's $1.2M to fill that bucket. But then at 62 you claim SS, which combined brings in $50K/yr, so you really only need $50K/yr to fill the gap. So assume $50K/yr for 15 years, until the statistics say that one of you may die -- that's $750K. Then from 77-on, your travel expenses go way down, but your medical expenses go up, so maybe now you need $80K/yr and have $25K in SS, so now your delta is $55K/yr -- etc. etc. etc. Once you figure out how much you need in each bucket, you present-value that to tell you how much you need to have today to fill that bucket -- starting from the furthest-out time period. So maybe you have a bucket from 77-95, and you calculate you need $900K to fill that bucket. But that's many years away -- so the reality is that as of today, if you have say $150K, you already have enough to grow into $900K in 25-30 years. So that bucket is filled -- check. Then move to the next one. And when you get to the point where your current savings is sufficient to fill all of your buckets, you are FI and can RE whenever you want.
I found this very helpful, because it told me that I needed quite a bit less than the 4% rule did (we have 2 SS + a pension that will be more than adequate to cover any normal-person's lifestyle needs, so the "real" issue is just covering the time between now and 67-70, with a little extra on the side in case of LTC issues). And it was most effective for my DH, who had previously believed we needed some ridiculous 8-figure pot to retire in the style to which he'd like to become accustomed. ;-)