I was thinking about the amount of money required to pay a mortgage after FIRE. Any standard category of spending would require 25x yearly outlay saved to cover using the 4% rule, but mortgages are not indexed to inflation. So if I'm thinking about this correctly, you would need less than 25x coverage since your effective payment "decreases" over time.
It seems to me that you can reverse the formula for calculating your monthly mortgage payment to find out how much you need to save.
Instead of:
A = P * r*(1+r)^n
(1+r)^n - 1
where A = monthly payment, P = initial loan principal, r = interest rate (per month), and n = total number of months remaining on the loan
you can flip it to:
P = A * (1+r)^n - 1
r * (1+r)^n
where A = monthly payment, P = initial stash needed to pay off loan over time, r = ASSUMED investment return, not adjusted for inflation (per month), and n = total number of months remaining on the loan
Is this right?
**My searches came up empty for this topic, so please direct me to the right thread if this has been discussed already.**