You might want to be careful setting the old loan to 0. This could be a problem if you refinance for more than $750k and the original loan originated before Dec 15, 2017. In this case, you are allowed to deduct interest on the amount up to the old loan payoff. For example, original loan in 2016 was $900,000 and you refinanced loan with no cashout for $780k in 2020. You want $780k as the amount on 12/31/2020 or date of payoff (which the form states). The $780k becomes your new mortgage interest deduction limit, not $750k.
