how do you calculate mortgage interest deduction on a cash out prior to 2018?

[menat]

With the new language in TurboTax 2018 on Cash Out - it occurred to me I had not filed the Mortgage Interest correctly in previous years, and need to Amend 2016 and 2017.   What I don't know is how to calculate it for those years specifically.   I read the equation to take the amount of the mortgage used specifically for the primary house, add $100K and divide that by the whole mortgage amount giving a %, which is then multiplied by the Mortgage Interest paid during the year - which is pretty straight forward.   But noticed in the 2018 version, it didn't do that.   It only asked what the amount was for the primary house, and the end of year balance on the mortgage, and used that %.   

So for 2016, 2017 - which way do you calculate - by the original full mortgage , or by the end of year mortgage balance?   Hope this makes sense...   Thanks!!

[Carl]

"So for 2016, 2017 - which way do you calculate - by the original full mortgage , or by the end of year mortgage balance? "

Neither. All mortgage interest paid prior to Jan 1, 2018 has no limits. It's all 100% deductible if the loan was established prior to Dec 15th, 2017.

[menat]

Carl - that's definitely not what I'm reading out there.   But I have a more specific question now open (no need for further response)  thanks 🙂