take the ending balance for each month for each mortgage from your servicer statements to strike an average over the 12 months. a lot of months may be zero.
see publication 936 / middle of page 13
Statements provided by your lender. If you receive monthly statements showing the closing
balance or the average balance for the month, you can use either to figure your average balance
for the year. You can treat the balance as zero for any month the mortgage wasn't secured
by your qualified home.
run the average on each mortgage through the table on page 12 and complete the ENTIRE page.
it wasn't clear to me when you sold the 1st house, but if that was simultaneous with the purchase of the new home, your average for the year is going to be under $750,000 and all the interest will be deductible.
