Get your taxes done using TurboTax

TurboTax is calculating your deductible mortgage interest by taking the loan limit ($750,000 in your case) divided by your outstanding loan balance. Then, that percentage is multiped by your mortgage interest as shown on your 1098. This amount is then deducted on your tax return on Schedule A. 

To show you an example of how it is calculating, I created a mock-up return using these figures: $50,000 in mortgage interest, $900,000 in outstanding principal and a mortgage origination date of 12/31/2018. The limit in this case was also $750,000. I have attached a picture of part of the deductible home mortgage worksheet.