I ended up creating a spreadsheet to replicate the worksheet in Table 1 of IRS Publication 936 (2020). I also used the spreadsheet to evaluate the different methods for reporting the average balance as outlined in 936. This resulted in a larger deduction than using the outstanding mortgage principal reported by my banks on the 1098 form. The banks provided the balance at the beginning of the year on line 2 of the 1098 form.
I then multiplied the interest paid to each bank by the fraction in line 14 of the Table 1 worksheet to get the allowed interest deduction for each institution. I also multiplied the average balance for each bank by the same fraction in line 14. These were then the inputs into Turbo Tax. Turbo Tax accepted this inputs without difficulty. The debt totaled $750,000 so there were no hiccups.
