What do you mean by "allocating it over prior years in turbotax"? You don't do anything with your tax returns for the prior years. The first thing you do is enter your SSA-1099 in TurboTax the same as if you did not get a lump-sum payment. You enter the full amount from box 5 of the SSA-1099. You don't reduce it or adjust it. After you enter the SSA-1099, the next screen will ask you whether you received a lump-sum payment. Select Yes, then go through the subsequent screens for one of the years for which you got a lump-sum payment. When you get to the "Lump-Sum Social Security Payments Summary," if you have a lump sum for another year, click the button that says "Add Another Lump Sum." Repeat the process for each year that you got a lump-sum payment for. After you have entered all the years, click Done on the summary screen. TurboTax will determine whether the Lump-Sum Election will save you any money, and will fill out your Form 1040 appropriately.
You don't make any entry or adjustment on Form 1040 line 5a. TurboTax will enter the proper amount for you. Line 5a on your Form 1040 will be the same as box 5 on your SSA-1099.
See the following FAQ for more information about lump-sum Social Security payments.
What is a lump-sum Social Security payment?