The OPM is known for doing this.
You should leave box 2a blank. If you had "basis" in your pension, then you will have to go through the Simplified Method in TurboTax to figure out what your taxable income is. "Basis" is created when you make after-tax contributions to your pension.
If you never made after-tax contributions to your pension, then box 2a would be the same as box 1.
So the answer to your question is it depends. But don't worry about you calculating the box 2 amount. As I noted above, box 2a is either equal to box 1 or TurboTax will use the Simplified Method to calculate the box 2a value. Just go through the 1099-R interview and see what happens.