The commission shall adopt rules that prescribe the allocation of costs for facilities, services or intangibles that are shared between regulated and unregulated activities of a utility or an affiliated interest as defined in section 707, subsection 1, paragraph A. Rules adopted by the commission may not establish a presumption with regard to the value of good will used by an affiliated interest in those cases where the business venture of the affiliated interest is regulated by the commission. Rules adopted pursuant to this section are major substantive rules as defined in Title 5, chapter 375, subchapter II?A. [PL 1999, c. 158, §2 (AMD).]