What figure to enter into the year box for a lump sum payment from Social Security which is designated "for other tax years"?

There should be a detail section on your SSA-1099 form that shows a breakdown of the amount by year.  Use that breakdown to report the lump sum amount one year at a time.  You may need to enter information for more than one year.  

Take a look at the following TurboTax article for more information:  What is a lump-sum Social Security payment?

Thank you for taking time to reply. I appreciate your effort. Those directions are the initial steps which cover smaller amounts with assigned years , but does not explain what to do with a larger amount "for other tax years." One of Turbo support staff figured out that the larger amount needed to be evenly portioned out over more years. Still waiting to see if that method passes Turbo review for errors.