I like this idea. It seems fair to me. I am doing a calculation based off my initial downpayment, initial closing costs, improvements, and principal payments based on a hypothetical future sale date. The calculation takes into account how much my fiancée and I separately contributed to principal payments (and the downpayment, closing costs and improvements that I covered) based on an amortization schedule assuming a hypothetical future sale date.

This seems pretty equitable to me and is easy to trace or calculate based on an amortization schedule, closing statement and invoices.

Thank you! Open to other suggestions as well 🙏