nope. you are in essence deducting interest on about 1.2 M in mortgage debt when your cap is $750K
see IRS pub 936 table 1. the cap applies to total mortgage debt. there is no separate limit for pre and post
see line 11 of deductible home mortgage interest worksheet
put another way say your pre12/15/2017 average loan balance was $1,000,000
and post average loan balance was $750,000
under your method you would be deducting interest on 1,750,000
also, no basis for adding in the over $4K in interest that wasn't actually paid. using the average monthly loan balance because is higher than the overall average will automatically give you a higher deduction. you can't count the time this mortgage didn't exist
what do I think the proper deduction should be - assuming average monthly mortgage balances computed correctly
750,000/ (523709 {assumes monthly average to be consistent with the other} +1040946) * (16197+12295+418)
this should be consistent with the results using table 1 in the IRS PUB and with proper input into Turbotax
i tried both and they both came up with the virtually the same amount ($1 rounding difference)
