The outstanding mortgage balance you calculated as well as the other information you enter into TurboTax will go onto the TurboTax worksheets, but those are not reported to the IRS. The only number reported to the IRS is the deductible mortgage interest on Schedule A, they do not get a copy of the Form 1098 you are filling out. They can question how you generated that number, so you will want to keep any notes with your tax records to remind yourself how you derived the outstanding mortgage balance for each loan.
