29 CFR 4044.41: General valuation rules.

(a) Valuation of benefits—(1) Trusteed plans. The plan administrator of a plan that has been or will be placed into trusteeship by the PBGC shall value plan benefits in accordance with §§ 4044.51 through 4044.58.

(2) Non-trusteed plans. The plan administrator of a non-trusteed plan shall value plan benefits in accordance with §§ 4044.71 through 4044.75. If a plan is unable to satisfy all benefits assigned to priority categories 1 through 4 on the distribution date, the PBGC will place it into trusteeship and the plan administrator shall re-value the benefits in accordance with §§ 4044.51 through 4044.58.

(b) Valuation of assets. Plan assets generally will be valued at their fair market value as defined in § 4001.2 of this chapter. As appropriate, plan assets will be valued at their fair value in accordance with accounting principles generally accepted in the United States of America (U.S. GAAP).