My CSA 1099R box 2a: Taxable amount states Unknown. Anybody follow the 721 publication instructions?

I have chosen the Simple method as a civil servant retiree and read where I need to compute the taxable amount. I am asking about this for some guidance in a simpler answer if that is possible or have I fallen in to a rabbit hole? In typical government instructions, if one hasnt does this it sounds too thick the first time.

Does anybody have to do this yearly? I am guessing that once I get the simple answer then I can just perform this process every year. This is my first time coming across this. The main sticking point is the 'Enter your cost in the plan at the annuity starting date'. Where is this to be found? Also there is a monthly remittance to a former spouse and a message in the CSA 1099R stating a yearly gross annuity reduction for the remittance that the spouse is responsible for. My thinking is this statement and the box 2a Unknown are connected. The sad and frustrating thing about retirement is there is no real connection between the IRS, SSA, OPM, Medicare and health care providers.

Thanks in advance for any answers or guidance.