I understand what you're trying to do. Assuming you're modeling a 30-year mortgage and plan to die exactly 30 years after retirement with no money left, it sorta makes sense. The issue I see is that a mortgage is a contract with a fixed interest rate and payment while investments are variable. That volatility complicates the retirement calculations and makes numbers from the mortgage method quite a bit more rosy than reality.
well, I just put the term to 50 years or whatever. I also was saying you can put the interest rate lower(basically set it to what you think your SWR is). The benefit from what I have done is to adjust what I think the actual rate of return will be and to insert my own inflation rate(I like to use 4.5% for inflation). So typical market rate from last 10 years or whatever I happen to have researched minus inflation for things like food not just other consumer goods.
Really the reason I like this it to put in my own inflation adjusted rate as opposed to what the calculators say. I would say the inflation rate for things like food, some other staples, or a used car is closer to 4.5%. Computers or other things that get cheaper with technology or economies of scale or other business practices skew the inflation rate to look lower. Case in point; eggs, beef, or price of land.
The other thing that aggravates me is that it works backwards from what I want. If I am saving retirement funds at the same time as saving for down payments on rental properties then it is a little more complicated. Really, it is just a way to view it backwards from how the retirement calculator does.