I assume you are asking about a Social Security Lump Sum payment for past year(s) benefits.
You need to enter the information from your Form SSA 1099 in the Turbotax area for entering Social Security Benefits received. Enter the information from Boxes 3, 4, 5 & 6. When Turbotax asks if any of these payments were for a lump-sum or were for previous years, answer Yes. Turbotax will then walk you through the process of entering the needed information for the previous years $$ payments, one year at a time. Your Form SSA 1099 should list the $$ amount paid in 2016 for 2015 or 2014 or other previous years. That information is listed in the area titled "Description of Amount in box 3".
You will need to have some information from your tax returns for those previous years.
You do not need to do any calculations, Turbotax does that for you. You can review TurboTax's calculation on the TT worksheets.