The ratio which the cost of the parts in paragraph (c)(1) of this section bears to the sum of the cost of such parts and the parts in paragraph (c)(2) of this section is applied to the lowest established wholesale price for which like combination articles are at the time of the taxable use being sold by the manufacturer or producer in the ordinary course of trade. The resulting amount is the constructive sale price for the taxable article on which tax is to be computed. The cost of the common parts is allocable to the parts in paragraphs (c) (1) and (2) of this section in the same ratio, and, therefore, need not be taken into account in the computation since the inclusion and allocation of the cost of such parts in the determination would not result in a different ratio. In determining the lowest establishment wholesale price for the combination article, there shall be included and excluded, as applicable, the charges and readjustments specified in sections 4216(a) and 6416(b)(1) of the Code, as in effect at the time tax liability on the use of the taxable article is incurred, and the regulations thereunder contained in this subpart and subpart L of this part (§§ 53.91-53.94 and §§ 53.173-53.176). The tax applicable to the use of the article for which a constructive sale price has been computed is not affected by any charges or readjustments of the price for which the nontaxable combination article is sold, whether by reason of the return or repossession of the nontaxable article or its covering or container, or by a bona fide discount, rebate, allowance, or other factor.
27 C.F.R. § 53.115