Average Mortgage Balance

I have a mortgage balance in January of of the year of $1,000,000. Paid down a significant part of it in February (still the same mortgage). So the current balance is less than $750,000. But the average balance is far less than beginning of year + end of year balance divided by 2.  I can calculate the average balance by averaging all of the months (accepted by IRS) but how do I get this into TurboTax to calculate the fair mortgage interest deduction (desktop premier version)?