Retiring in 2017 made 2017 your first RMD year for the TSP.  Assuming that you do not have a spouse more than 10 years younger who was the sole beneficiary of your TSP, the portion of your TSP distribution that was RMD for 2017 was your traditional TSP balance on December 31, 2016 divided by 25.6  (The 25.6 factor is from the Uniform Lifetime Table for your age of 72 on your birthday in 2017.

(TurboTax only ask the amount of the distribution that was RMD to ensure that you did not roll over to another retirement account any portion of the distribution that was RMD.  If you did not roll any of this distribution over to another retirement account, it doesn't matter what amount you enter as the portion that was RMD.)