29 CFR 4044.73: Lump sums and other alternative forms of distribution in lieu of annuities.

(a) Valuation. (1) The value of the lump sum or other alternative form of distribution is the present value of the normal form of benefit provided by the plan payable at normal retirement age, determined as of the date of distribution using reasonable actuarial assumptions as to interest and mortality.

(2) If the participant dies before the date of distribution, but had elected a lump sum benefit, the present value shall be determined as if the participant were alive on the date of distribution.

(b) Actuarial assumptions. The plan administrator shall specify the actuarial assumptions used to determine the value calculated under paragraph (a) of this section when the plan administrator submits the benefit valuation data to the PBGC. The same actuarial assumptions shall be used for all such calculations. The PBGC reserves the right to review the actuarial assumptions used and to re-value the benefits determined by the plan administrator if the actuarial assumptions are found to be unreasonable.