(1) For purposes of ORS 317.391, the property factor is a fraction, the numerator of which is the average value of the taxpayer’s real and tangible personal property owned or rented and used in this state during the tax period and the denominator of which is the average value of all the taxpayer’s real and tangible personal property owned or rented and used during the tax period.
(2) Property owned by the taxpayer is valued at its original cost. Property rented by the taxpayer is valued at eight times the net annual rental rate. Net annual rental rate is the annual rental rate paid by the taxpayer less any annual rental rate received by the taxpayer from subrentals.
(3) The average value of property shall be determined by averaging the values at the beginning and ending of the tax period but the Department of Revenue may require the averaging of monthly values during the tax period if reasonably required to reflect properly the average value of the taxpayer’s property.