In working my way up to FIRE, I had four large segregated items:
1. My kids' college.
2. My house/mortgage.
3. Child support.
4. My FIRE stash.
I calculated that I would FIRE when some of my kids were in college and others were still requiring me to pay child support. If I paid on my mortgage according to schedule, I'd still also have a mortgage at that point.
Because those non-FIRE items were large chunks of my budget, I was very concerned that I would do the FIRE math incorrectly and leave my job based on a broken Excel spreadsheet, then discover that I didn't actually have enough.
So I did the following accounting / bucketing things:
1. I set up a separate "College" spreadsheet that calculated how much I had in the kids' college funds, how many years away their college would be, how much I had budgeted to provide, how much the costs would grow in the meantime, how much the money would grow in the meantime, etc. I then figured out how much underfunded I was. I then put money into those accounts until they were fully funded. So those accounts were for their college, and the other accounts were for other things.
2. With my mortgage, I just put everything extra on it until it was paid off. Then I could just look at my remaining expenses and work to fund 25x those forever, instead of 25x of those plus the mortgage balance. This probably wasn't the most efficient way of getting to FIRE, as I had a low rate mortgage and reasonably high risk tolerance for investing. Again, it simplified things to keep track of, because I didn't have to deal with escrow accounts, tax deductions, mortgage payments, etc.
3. With my child support, what I ended up doing in my retirement spreadsheet was subtracting the NPV of my remaining payments from my FIRE stash amount. I could have done it like my mortgage, but in the case of child support it doesn't work as well to try to prepay several years. When I put my numbers into cFIREsim or Firecalc I put in my current expenses including child support, then put in an offset amount for the reduced spending for next year when it will end.
I did FIRE 3 years ago, and things have worked out well so far. I did make one major error in my "college" spreadsheet, but that error resulted in over funding their accounts. So I've decided to distribute the extra to them as they graduate from college.