5 CFR 839.1119 - How is the actuarial reduction for TSP computed?
(a) The part of your TSP account on the date you retired that is Government contributions and earnings on those Government contributions forms the basis for the actuarial reduction. OPM will divide the Government contributions and earnings by the present value factor for your age (in full years) at the time you retired. OPM will then round the result to the next highest dollar amount, which will be the monthly actuarial reduction amount.
(b) If a survivor annuity is the only benefit that is payable, the present value factor for the survivor's age at the time of death is used. The survivor benefit is not reduced for TSP if the retiree's rate was reduced.