Pick a place you might want to live after you FIRE, and then research cost of living there. It doesn’t have to be where you end up, but it helps with estimates. Or pick three places do the research and average them out. A lot of averages for things like utilities, cost of fuel or transit, insurance for that area are available online. And of course you can look at house listings online to see housing costs.
I picked a LCOL area to do estimates and was kind of surprised that the house + transportation + utilities were actually higher than my large city, but MCOL, situation. Then again that’s comparing a one bedroom apartment and no car here to a 2-3 bed house with at least one (older) car there.