No, there's no way to "show your work" in TurboTax Online. You can replace the amount in Box 2 of Form 1098 with your calculated amount using the interest rate method. You would want to maintain your records for at least three years, as you always do in case the IRS were to question how you came up with the number. If the amount using this method is still high enough that your mortgage deduction is limited, TurboTax will ask you the amount of Outstanding Mortage Principal on January 1, 2022. You will want to use the same number that you calculated again because it is going to take the average of the two numbers.
