The amount of indebtedness so to be assumed shall be calculated on the basis of the proportion which the original cost of the buildings, grounds, furnishings, equipment, and additions thereto, situate in the proposed new district, bears to the original cost of the buildings, grounds, furnishings, equipment and additions thereto, situated in the original district. Said original cost shall be determined according to rules prescribed by the commissioner with the approval of the state board and in accordance with recognized accounting practices.
N.J.S. § 18A:8-7