To build off your example.
Someone spending 40,000
Stash 250,000
FIRE number 1,000,000
So today they are 25% funded. But they can't retire. Let's say they work 10 more years before pulling the trigger. Let's assume 2% inflation /year so 20% total for 10 years (simple math). That means, without any lifestyle creep, this person needs to replace 48,000 of spending, not 40,000. And thus their stash target has grown to 1,200,000 also. If they have hit their 1,000,000 target, they are not 100% funded, but only 83%.
Basically, however many "freedom days" you've bought now, they loose purchasing power with inflation just like anything else. You're investments, will out pass it, but you still need to take into account the time order difference.