If my CSF 1099R form has "unknown" in box 2a and 4-death benefit in box 7, what do I enter for the taxable amount in box 2a?

You let the program determine that. Be sure to pick the correct 1099-R type:  Standard 1099-R, CSA-1099-R, CSF-1099-R, RRB-1099-R.

 

Tax rules for annuity benefits.

Part of the annuity benefits you receive is a tax-free recovery of your contributions to the CSRS or FERS. The rest of your benefits are taxable. If your annuity starting date is after November 18, 1996, you must use the Simplified Method to figure the taxable and tax-free parts. If your annuity starting date is before November 19, 1996, you generally could have chosen to use the Simplified Method or the General Rule.