The taxable amount in box 2a is usually the box 1 amount unless you have after-tax contributions in the retirement plan and use the simplified method. If this is NOT the first year of receiving payments, then you should use the same method that was used last year - either the box 1 amount of the simplified method, using the carry forward simplified information from last year.
If box 2a is blank or UNKNOWN then there should be amount in box 9b to use with the simplified method. If no amount in 9b then contact OPM to find the account "basis".
**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.**
