29 C.F.R. § 4062.8

(a) Liability amount. If, pursuant to section 4062(e) of ERISA, an employer ceases operations at a facility in any location and, as a result of such cessation of operations, more than 20% of the total number of the employer's employees who are participants under a plan established and maintained by the employer are separated from employment, the PBGC will determine the amount of liability under section 4063(b) of ERISA to be the amount described in section 4062 of ERISA for the entire plan, as if the plan had been terminated by the PBGC immediately after the date of the cessation of operations, multiplied by a fraction—

(b) Example. Company X sponsors a pension plan with 50,000 participants of which 20,000 are current employees and 30,000 are retirees or deferred vested participants. On a PBGC termination basis, the plan is underfunded by $80 million. Company X ceases operations at a facility resulting in the separation from employment of 5,000 employees, all of whom are participants in the pension plan. A section 4062(e) event has occurred, and the PBGC will determine the amount of employer liability under section 4063(b) of ERISA. The numerator described in paragraph (a)(1) of this section is 5,000 and the denominator described in paragraph (a)(2) of this section is 20,000. Therefore, the amount of liability under section 4063(b) of ERISA pursuant to section 4062(e) is $20 million (5,000/20,000 × $80 million).