(1) In determining whether the activities of any company have been conducted within Montana beyond the protection of P.L. 86-272, the principle established in Appeal of Finnigan Corporation, 88-SBE-022 (8/25/1988) and Appeal of Finnigan Corporation, 88-SBE-22A, Opinion on Petition of Rehearing (1/24/1990), commonly known as the "Finnigan Rule," shall apply. When calculating the Montana apportionment numerators provided for in 15-31-306, 15-31-309, and 15-31-311, MCA, a group of corporations engaged in a unitary business as defined in 15-31-301, MCA, shall include Montana property, payroll, and receipts from all members of the unitary group as long as one or more members has nexus with Montana.