I am surprised no one has responded to your question. You probably figured it out yourself by now. But if you haven't: The percentage of your total interest that you can deduct is calculated (average acquisition debt)/(average mortgage balance). Acquisition debt is the balance of the loan that was used to buy or improve the the home. If this is the original mortgage (not a refinance) all of the mortgage is acquisition debt.
There several ways to calculate the average balances described on page 12 of Pub 936. You can add the beginning and ending balances together and divide by 2: ($900,000 + $880,000)/2 = $890,000. To figure the percentage of interest after applying the limitation, divide $750,000 by $890,000 and round the result to three decimal places: $750,000 / $890,000 = 0.843 = 84.3%
