My mortgage spreadsheet is more honest than my startup model

I keep a spreadsheet for our mortgage. Overpay early or invest the difference, that is the whole question, and I have rebuilt it maybe ten times.

Every version gives a different answer, and always because of one cell. Expected return. Change that one number and the answer flips. Everything else in the model is arithmetic I can check.

Runway is the same shape. The answer depends on a growth rate nobody knows, and the rest is addition.

The difference is that the mortgage sheet has no audience. Nobody sees it, so I never round anything in my favour. The startup model gets shown to people, and that is exactly where the definitions start drifting.

Do you keep a personal money model? And is it more honest than the one you show to investors? 🤔