(a) General rule. In accordance with the rules in this section, the plan sponsor shall determine the amount of unfunded vested benefits to be reallocated and shall fully allocate those unfunded vested benefits among all employers liable for reallocation liability.

(b) Amount of unfunded vested benefits to be reallocated. For purposes of this section, the amount of a plan's unfunded vested benefits to be reallocated shall be the amount of the plan's unfunded vested benefits, determined as of the mass withdrawal valuation date, adjusted to exclude from plan assets the value of the plan's claims for unpaid initial withdrawal liability and unpaid redetermination liability that are deemed to be uncollectible under § 4219.12(c)(1) or (c)(2).

(c) Amount of reallocation liability. An employer's reallocation liability shall be equal to the sum of the employer's initial allocable share of the plan's unfunded vested benefits, as determined under paragraph (c)(1) of this section, plus any unassessable amounts allocated to the employer under paragraph (c)(2), limited by section 4225 of ERISA to the extent that section would have been limiting had the employer's reallocation liability been included in the employer's initial withdrawal liability. If a plan is determined to have no unfunded vested benefits to be reallocated, the reallocation liability of each liable employer shall be zero.

(1) Initial allocable share. Except as otherwise provided in rules adopted by the plan pursuant to paragraph (d) of this section, and in accordance with paragraph (c)(3) of this section, an employer's initial allocable share shall be equal to the product of the plan's unfunded vested benefits to be reallocated, multiplied by a fraction—

(i) The numerator of which is the yearly average of the employer's contribution base units during the three plan years preceding the employer's withdrawal; and

(ii) The denominator of which is the sum of the yearly averages calculated under paragraph (c)(1)(i) of this section for each employer liable for reallocation liability.

(2) Allocation of unassessable amounts. If after computing each employer's initial allocable share of unfunded vested benefits, the plan sponsor knows that any portion of an employer's initial allocable share is unassessable as withdrawal liability because of the limitations in section 4225 of ERISA, the plan sponsor shall allocate any such unassessable amounts among all other liable employers. This allocation shall be done by prorating the unassessable amounts on the basis of each such employer's initial allocable share. No employer shall be liable for unfunded vested benefits allocated under paragraph (c)(1) or this paragraph to another employer that are determined to be unassessable or uncollectible subsequent to the plan sponsor's demand for payment of reallocation liability.

(3) Contribution base unit. For purposes of paragraph (c)(1) of this section, a contribution base unit means a unit with respect to which an employer has an obligation to contribute, such as an hour worked or shift worked or a unit of production, under the applicable collective bargaining agreement (or other agreement pursuant to which the employer contributes) or with respect to which the employer would have an obligation to contribute if the contribution requirement with respect to the plan were greater than zero.

(d) Plan rules. Plans may adopt rules for calculating an employer's initial allocable share of the plan's unfunded vested benefits in a manner other than that prescribed in paragraph (c)(1) of this section, provided that those rules allocate the plan's unfunded vested benefits to substantially the same extent the prescribed rules would. Plan rules adopted under this paragraph shall operate and be applied uniformly with respect to each employer. If such rules would increase the reallocation liability of any employer, they may be effective with respect to that employer earlier than three full plan years after their adoption only if the employer consents to the application of the rules to itself. The plan sponsor shall give a written notice to each contributing employer and each employee organization that represents employees covered by the plan of the adoption of plan rules under this paragraph.