(1) 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 real and tangible personal property owned or rented and used during the tax period.

(2) Property owned is valued at its original cost. Property rented is valued at eight times the net annual rental rate. Net annual rental rate is the annual rental rate paid by the taxpayer.

(3) The average value of property shall be determined by averaging the values at the beginning and end of the tax period, but the Tax Commissioner may require the averaging of monthly values during the tax period if reasonably required to reflect properly the average value of the property.