If the apportionment provisions do not fairly represent the extent of the taxpayer's business activity in this Commonwealth, section 401(3)2.(a)(18) of the TRC (72 P. S. § 7401(3) 2.(a)(18)) may be utilized on a case-to-case basis to effectuate an equitable apportionment by any of the methods provided therein.
61 Pa. Code § 153.43