In the case of an interstate carrier, the rolling stock used in the Commonwealth shall be deemed to be that portion of the total rolling stock, owned or operated on the public highways of the Commonwealth, multiplied by a fraction wherein the numerator is the total vehicle miles traveled by such rolling stock in the Commonwealth and the denominator is the total vehicle miles traveled both within and without the Commonwealth on such operations as are related to the Commonwealth.
Va. Code § 58.1-2655