Variables like location or single/couple/family make a difference and are hard to reconcile across the board obviously. I don't think using WR is a good measure as it is more a measure of risk and certainty, possibly wealth too, but not so much spending. I mean if I have a 3% WR and spend $20k I am pretty sure most would not consider that fat fire (spartana excluded), at least not in the US.
Maybe WR would be indicative if it was based on just discretionary spending or something like of my 4% WR about 75% is for discretionary.
Maybe an easier more broad approach is to base it on some % of median household income for one's area. Median for my town is $95k so would $100k make my family fat fire (or should I base it more on my neighborhood that has a med hh Inc of $150ish) On one hand, I wouldn't have to pay payroll taxes or set money aside for retirement but I wouldn't have an employer picking up a good chunk of my health care. Also, fed/state taxes could be less if more from dividends and cap gains and principle drawdown or higher if all from 401k/non-roth IRA, which also impacts Healthcare.
Maybe 1 to 1.5x median household income for where you live is Fat FIRE
Maybe a multiple of one's location median household income is determinant
>1.5x is hoggish fire
1.0 to 1.5x is fat fire
0.5x to 1.0x is reg fire
< 0.5x is lean fire
A couple at or above the med hh income spending level iwithout work feels pretty fat. And if doing that in a HCOL then there is the add flex to go to lower COL and be fatter.
IDK, just a thought experiment, as I said I think $100k is a lot but maybe I have it wrong or right.