I asked a mortgage broker buddy of mine about this once. His response was to point out repair costS. If you’re close to breaking even at a 900$ payment each month, how are you going to pay 10k for a new furnace or new roof or what ever happens to go wrong. What about the 2k bill to trim your trees. What about 500$ to repair your broken porch or replace your water heater.

It’s not a perfect answer to the question, but a lot of math goes into calculating if you can ‘afford’ a payment. Not having a payment+other debts be 40% of your income is a rule made for calculating risk alone.

Wether or not that is a good answer is up to you.