If I do not live in a property I own, I need to hold assets or be earning income that cover my expected rent. According to the criteria I'm using as a guide for complete financial independence, I would need assets equal to 300 multiplied my expected rent to provide that income. If I can buy a property I want to live in for less than that figure, then, bingo, doesn't it make more sense for me to buy?
Have I stumbled upon the simplest and most convincing FIRE-friendly deciding factor regarding buying or renting a place to live? Is this something I missed in the FIRE literature or possibly have seen and forgotten about?
What you're missing is you're assuming you need assets to cover that expense indefinitely, when the mortgage will end.
For example, picture someone who owes 20k left on a mortgage that costs them 10k/yr, spends 30k/yr without counting the mortgage. Their total cost of living, right now, is 40k (30k + 10k mortgage). They calculate that, for a 4% WR, they'd need 1MM to cover that 40k spending.
The reason that this is wrong is that they don't need to cover that 40k spending forever. The mortgage, at a balance of 20k, will be paid off in 2 years (at the 10k/yr). After that, they just need to cover the 30k spending. So they correct amount they need to FIRE is 25x30k expenses = 750k + the 20k mortgage balance = 770k. This will leave them with, in two years (ignoring market movements in the meantime), 750k to cover their 30k spending at a 4% WR.
770k is much less to save than 1MM, and it's because they don't have to cover that mortgage payment (10k) indefinitely--that expense ends.
So even if they're keeping that extra 20k invested, rather than paying off their mortgage now, because it makes them more money, they don't need to save more than that mortgage balance.
Your "save 300x the payment to cover the mortgage, and use that as a comparison" doesn't work, for that reason. Does that make sense? :)
neogodless posted the right answer, but there's a ton of other irrelevant posts, which are important considerations for FIRE (like house upkeep, and other expenses) but don't actually address the original question, of "does this 300x calculation to compare make sense." neogodless's answer is perfect, but hopefully my explanation helped clarify it. No, 300x comparison doesn't work, unfortunately.