In any case in which, in any plan year following a partial withdrawal under section
1385
(a)(1) of this title, the number of contribution base units with respect to which the employer has an obligation to contribute for such year equals or exceeds 110 percent (or such other percentage as the plan may provide by amendment and which is not prohibited under regulations prescribed by the corporation) of the number of contribution base units with respect to which the employer had an obligation to contribute in the partial withdrawal year, then the amount of the employer’s partial withdrawal liability payment for such year shall be reduced pro rata, in accordance with regulations prescribed by the corporation.