OPM survivor annuity taxable amount calculator

[jasdlm]

Does anyone know how to calculate the non-taxable portion of a survivor annuity (OPM)?  I can find plenty of references to the calculation for the participant, but does it change at the death of the participant since the survivor benefit is 55% of the original annuity but the after-tax contributions haven't changed?  Am I over thinking this?  Under thinking this?  Smoking crack?

Thanks.

[Lee B]

"A. Yes, they are. However, if there are any unexpended retirement contributions in the late spouse’s account, a portion of the annuity would be tax-free. For more information, go to https://www.irs.gov/publications/p721/index.htm"