N.J. Stat. Ann. § 18A:13-68

The county superintendent shall calculate the amount of indebtedness to be assumed on the basis of the proportion which the replacement cost of the buildings, grounds, furnishings, equipment, and additions thereto of the all purpose regional district situated in the withdrawing municipality bears to the replacement cost of the buildings, grounds, furnishings, equipment and additions thereto situated in the entire all purpose regional district. The replacement cost shall be determined according to rules prescribed by the Commissioner of Education with the approval of the State board and in accordance with recognized accounting practices.