29 CFR 4206.5 - Amount of credit in plans using the modified presumptive method.
(a)General. In a plan that uses the modified presumptive method described in section 4211(c)(2) of ERISA, the credit shall equal the sum of the unamortized old liabilities determined under paragraph (b) of this section, multiplied by the fractions described or determined under paragraph (c) of this section. When an employer's prior partial withdrawal liability has been reduced or waived, this credit shall be adjusted in accordance with § 4206.8.
(b)Unamortized old liabilities. The amounts described in this paragraph shall be determined as of the end of the plan year preceding the withdrawal for which the credit is being calculated, and are the employer's proportional shares, if any, of -
(1) The plan's unfunded vested benefits as of the end of the last plan year ending before September 26, 1980, reduced as if those obligations were being fully amortized in level annual installments over 15 years beginning with the first plan year ending on or after such date; and
(2) The aggregate post-1980 change amount determined under section 4211(c)(2)(C) of ERISA as if the employer had completely withdrawn in the year of the prior partial withdrawal, reduced as if those obligations were being fully amortized in level annual installments over the 5-year period beginning with the plan year in which the prior partial withdrawal occurred.
(c)Employer's allocable share of old liabilities. The sum of the amounts determined under paragraph (b) are multiplied by the two fractions described in this paragraph in order to determine the amount of old liabilities that was previously assessed against the employer.
(2) The second fraction is a fraction, the numerator of which is the amount of the liability assessed against the employer for the prior partial withdrawal, and the denominator of which is the product of -
(i) The amount of unfunded vested benefits allocable to the employer as if it had completely withdrawn as of the date of the prior partial withdrawal (determined without regard to any adjustments), multiplied by -