Mont. Code Ann. § 15-23-105

The department shall apportion the value of property assessed under 15-23-101, 15-23-205, or 15-23-403, other than railroad car company property, among the counties in which such property is located. Apportionment shall be on a mileage basis or on the basis of the original installed cost of the centrally assessed property located in the respective counties. If the property is of such a character that its value cannot reasonably be apportioned on the basis of mileage or on the basis of the original installed cost of the centrally assessed property located in the respective counties, the department may adopt such other method or basis of apportionment as may be just or proper.