Partially because I'm still almost 14 years away from the earliest time I can claim SS, I'm not comfortable with anything less than a 100% success rate according to Firecalc (and not counting SS).
Rather than figuring out what I needed to live on and using that figure to calculate how big my stash needed to be, I approached it a little backward. After getting laid off and realizing that I didn't really want to work again, I looked at my stash, used Firecalc to figure out a 100% success WR and found a way to live off less than that.
It's a rather low sum and I'm only 8 months into this experiment, but I'd rather live on a low income when I'm young and be be able to increase it as I get older, than have to do it the other way around.
Besides, it's kind of fun keeping to this strict budget (I'm weird that way).