Generally the box 2a (or box 1 if 2a is blank) is the taxable amount unless part of the distribution is from after-tax contributions (that might be in box 5 or box 9). Unless this is the *first* year of receiving pension payments then you use the same method that was used in 2018.
If this is the first year and there is after-tax money in the account that should not be taxed again so the simplified method should be used to reduce the tax. Use your own records for the after tax money or get it from the plan administrator (if the administrator kept track).
**Disclaimer: This post is for discussion purposes only and is NOT tax advice. The author takes no responsibility for the accuracy of any information in this post.**
