I am going to modify my earlier statement.
There actually is a statement on how to allocate when there is a mixed use mortgage.
https://www.irs.gov/pub/irs-pdf/p936.pdf
on a mixed use mortgage, there is a prescribed paydown order. you don't have a choice: (Page 12 - right side). If you decide to make a $50k paydown, it has to go against the non-aquisition debt first.
"Figure the balance of that category of debt for each month. This is the amount of the loan proceeds allocated to that category, reduced by your principal payments on the mortgage previously applied to that category. Principal payments on a mixed-use mortgage are applied in full to each category of debt, until its balance is zero, in the following order.
a. First, any home equity debt not used to buy, build, or substantially improve
the home.
b. Next, any grandfathered debt.
c. Finally, any home acquisition debt. "
