I disagree and believe that TurboTax is doing the wrong calculation. If you follow the official IRS publication 936, it includes both a worksheet and examples on page 13 on how to calculate the "Average Balance". Line 7 of the worksheet says "... Enter the average balance of all your home acquisition debt incurred after December 15, 2017 ..". The key take for me is the word "ALL" and if you are like where I had 3 mortgages on the same property with the principal paid down, I need to do this calculation for each. I used the "Interest paid divided by interest rate method." and did that for each 1098 and then added them up to come up with my average balance. The calculation was simple, where you divide the total interest paid by the interest rate on that mortgage. I took that number and completed the worksheet to come up with the number that should be used on Schedule A with form 1040. Now I am trying to figure out how to best manually file and apply these numbers, but this I believe is the correct and easiest way to calculate the number needed.
https://www.irs.gov/pub/irs-pdf/p936.pdf
