OK, now that I have that silly work stuff done, to follow up. You seem a bit like me, in that I am intimidated by big fancy spreadsheets; I mean, sure, they can be highly precise, but if I'm not going to take the energy to learn how to get all the inputs right, then how does that help?
If that sounds like you, don't let the perfect be the enemy of the good. If you want a somewhat simpler approach, here's an idea I picked up here several years ago -- it's a "bucket" method, but the "buckets" are different than what you're looking at.
First thing you do is to lay out a basic chronology of how you see your income and expenses changing, starting from the day you FIRE through until the day you die. So maybe if you retire at 52, you estimate you'll need $40K/yr then; at 58, the kids will all be independent so you can drop down to $35K/yr; then at say 62 you take SS, which will provide you $18K/yr; then at 75 you assume your medical expenses go up and you need $40K/yr; etc. -- you just map that out in a very big-picture way.
Then you group each set of stable years together and calculate out the money you will need to cover your expenses during each of those chunks of years. So from 52-58, you have $40K x 6 years = you will need around $240K by the age of 52 to get you to age 58;* 58-62 = $35 x 4 years = you need $140K by the time you hit 58 to make it to 62; 62-75 you need $35K/yr but you are now getting $18K SS, so the remaining need is only $17K/yr x 13 years = you need $221K by age 62 to make it to 75; etc. all the way out.
So now you have a list of buckets of money you will need and the date by which you will need them. Now comes the fun part: now you start applying your current savings to those buckets, starting from the furthest one out (the "XX until death" one). But here's the thing: we all know $1 today is worth a lot more than $1 in 30 years, right? And your job is to figure out how many of today's dollars you will need to fill each bucket as of the date you need it. So say that you've assumed you will live to 100, and you assume that from 87-100 you'll need $50K/yr to cover extra medical care. So 40 years from now, you will need to have $50K x 13 years = $650K. But that doesn't mean you need $650K now. You need to present-value that $650K. And you don't need too much money now to cover $650K 40 years in the future. So I just pulled up The Google and found a present-value calculator, and it is telling me that if I set aside $63K now and get 6% returns, in 40 years I'll have $650K. So guess what? That bucket is already filled -- you have plenty of money invested already to cover your last 13 years.
Do that same exercise for every bucket, and as you go, mentally subtract that chunk of money from your total investments. So you have $1.25M now; you need say $65K of that for those final 13 years, which leaves you $960K to cover those earlier buckets. Then you do the next-farthest bucket, pop out that present-value calculator, and subtract that from the remaining assets you have. If you make it all the way back to 52 without running out of money, then you're already good to FIRE at 52, even without saving another penny, and maybe you can look at FIRE even sooner. But if you run out of $ before you have all the buckets filled, then you know what that gap is, and you can compare it to the additional amount you plan to save over that period of time to see if you're on track.
I have found this particularly useful because the 4% rule doesn't count for big variations in income and expenses over time (we will likely still be paying college tuition when we quit, and we're certainly not planning on that continuing forever!). IME, doing the math this way tells me I need a lot less money than the 4% rule says.
*Note this is not actually true, because of course your investments will continue to grow over those years. But I am going for "easy" more than "precise."
