29 U.S. Code § 1164 - Applicable premium
The term “applicable premium” means, with respect to any period of continuation coverage of qualified beneficiaries, the cost to the plan for such period of the coverage for similarly situated beneficiaries with respect to whom a qualifying event has not occurred (without regard to whether such cost is paid by the employer or employee).
An administrator may not elect to have subparagraph (B) apply in any case in which there is any significant difference, between the determination period and the preceding determination period, in coverage under, or in employees covered by, the plan. The determination under the preceding sentence for any determination period shall be made at the same time as the determination under paragraph (3).
The determination of any applicable premium shall be made for a period of 12 months and shall be made before the beginning of such period.