I'm not quite following (I thought I did, then I reread it and am not sure).
Here's what I'm saying: Let's say you'd be happy with $X spending and Y% success rate. Put in that $X spending, your AA, and the Y% into the "Maximum yearly/initial spending for success rate" option, and find out the stache amount you need.
then you just need to figure the time to FIRE based on your current amount saved, annual savings amount, and estimated rate of return, pretty basic math.
If you use the "
Minimum yearly/initial
portfolio for success rate" option, you're finding the least amount of stashe you need to retire at your desired spending level
for the exact retirement year listed. The "pretty basic math" that you speak of only applies to the very specific scenario where 100% of your savings/income is coming in pre-retirement, then completely stops upon your retirement date, and then all of your expenses are exactly the same throughout retirement (if you want to be accurate about it). That is the sort of math that MMM uses in the Shockingly Simple Math behind early retirement.
However, in reality, we have different income streams that will be there before AND after retirement, we have one-time expenses, we have expenses or income streams that will be on specific years regardless of the retirement year, etc etc. So, cFIREsim does a year-by-year calculation using those inputs to figure out your retirement path. That means that the "simple math" version can't really optimize to find your FI date.
Consider the attached photo. What I'm talking about, is implementing a "Investigate FI date" feature that has a little checkbox pop up next to every income/savings/expense input. If you click that checkbox for an input, you're telling cFIREsim that this is a strictly pre-retirement input (it ends when retirement happens). If you leave it unchecked, that input will be simulated no matter when you retire. In the photo, the 401k would be a pre-retirement only input, but all of the rest would not be. This would mean that moving the retirement date around would cause some different issues that can't be guessed with the simple math version.
Sorry if that was long-winded.