The value of a lump sum benefit which is not covered under § 4044.73 or § 4044.74 is equal to—

(a) The value under the irrevocable commitment, if an insurer provides the benefit; or

(b) The present value of the benefit as of the date of distribution, determined using reasonable actuarial assumptions, if the benefit is to be distributed other than by the purchase of the benefit from an insurer. The PBGC reserves the right to review the actuarial assumptions as to reasonableness and re-value the benefit if the actuarial assumptions are unreasonable.