US Govt OPM Survivor Annuity 1099 Taxable Amount

My Mom has a OPM survivor's annuity and I've always done her taxes.  Box 2a has always been UNKNOWN since my Dad's passing 11 years ago.  TurboTax has always been confusing to me with regards to this pension.  She gets regular monthly payments and I've always just reported everything she receives as RMD.  I also just considered the full amount as the taxable amount.  The questions seem to have changed this year...

- leave taxable amount in 2a blank

- TurboTax asks me a few questions later that they noticed I left it blank and that is OK

- TurboTax then asks if the Taxable Amount in 2a was used in years passed.  I say no and it comes up with Simplified Method and General Rule.  I've always chosen General Rule in the past and the full amount is populated by TurboTax OR I might have even put in the full amount in 2a. Have I screwed all this up over the years?  FYI Mom is nearly 100 years old and my Dad probably started receiving the pension in about 1987 and my he passed in 2013 so it became a survivor's pension.  I think when he was still living there was a formula used over a number of years which excluded a portion of his pension income but that was all used.