From Publication 936:

A mixed-use mortgage is a loan that consists of more than one of the three categories of debt (grandfathered debt, home acquisition debt, and home equity debt). For example, a mortgage you took out during the year is a mixed-use mortgage if you used its proceeds partly to refinance a mortgage that you took out in an earlier year to buy your home (home acquisition debt) and partly to buy a car (home equity debt).
Complete lines 1, 2, and 7 of Table 1 by including the separate average balances of any grandfathered debt and home acquisition debt (determined by the date the debt was acquired) in your mixed-use mortgage. Don’t use the methods described earlier in this section to figure the average balance of either category. Instead, for each category, use the following method.
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.
First, any home equity debt not used to buy, build, or substantially improve the home.
Next, any grandfathered debt.
Finally, any home acquisition debt.
Add together the monthly balances figured for b and c in (1).

If the principal payments are applied to home equity debt first (which is not deductible), how do you come up with your percentage result? Can you cite a reference for this?