29 C.F.R. § 4211.4

(a) The numerator of the allocation fraction, with respect to a withdrawing employer, is based on the “sum of the contributions required to be made” or the “total amount required to be contributed” by the employer for the specified period. For purposes of these methods, this means the amount that is required to be contributed under one or more collective bargaining agreements or other agreements pursuant to which the employer contributes under the plan, other than withdrawal liability payments or amounts that an employer is obligated to pay to the plan pursuant to section 305(e)(7) of ERISA or section 432(e)(7) of the Code (automatic employer surcharge). Employee contributions, if any, shall be excluded from the totals.

(b) The denominator of the allocation fraction is based on contributions that certain employers have made to the plan for a specified period. For purposes of these methods, and except as provided in § 4211.12, “the sum of all contributions made” or “total amount contributed” by employers for a plan year means the amounts considered contributed to the plan for purposes of section 412(b)(3)(A) or section 431(b)(3)(A) of the Code, other than withdrawal liability payments or amounts that an employer is obligated to pay to the plan pursuant to section 305(e)(7) of ERISA or section 432(e)(7) of the Code (automatic employer surcharge). For plan years before section 412 applies to the plan, “the sum of all contributions made” or “total amount contributed” means the amount reported to the IRS or the Department of Labor as total contributions for the plan year; for example, for the plan years in which the plan filed the Form 5500, the amount reported as total contributions on that form. Employee contributions, if any, shall be excluded from the totals.