REF: 2017 State Refund Worksheet and it'sFed Form 1040 Info Smart Wksht: Am I to complete Item D which is my 2016 sales tax deduction that I COULD have taken in 2016?

[reallaw]

Item D under the "2016 Federal Form 1040 Information SMART Worksheet states "Enter the Deduction for general sales tax that could have been taken in 2016".  In 2015 I used the Sales Tax Deduction and in 2016 I used the State Refund Deduction....TT picked up the amount of my 2016 Refund correctly but TT did not complete the Item D which ask for the possible 2016 Sales Tax deduction that I could have elected to take, if I had wanted to.  I looked on the IRS 2016 Sales Tax Tables and arrived at a correct amount to place into Item D .... However, as it increased our Federal Refund amount by $200 (+-) it made me nervous....so I need to know if I was correct in completing Item D with the amount from the IRS 2016 Sales Tax Table.  Thanks!!

[CherylW]

Turbo Tax tries to recapture the benefit you got in 2016 by taking the state income tax deduction.

If you didn't take the sales tax deduction in 2016, leave line D blank. 

[reallaw]

Why would TT ask for the amount of 2016 Sales Tax "that I could have taken" on Item D?  In the "instructions" TT tells why right after the instruction for Item D.   It states: "Why the program is asking for this amount: The program needs this information to properly compute the tax benefit amount. Only the excess of the income tax deduction over the sales tax deduction yielded a tax benefit in 2016. This amount is used in the computation of refigured itemized deductions on line 7 of the worksheet."
 If I complete Item D it reduces my 2017 tax but I do not know why...and I want to be sure. Thanks for your response!!!