The payment of all subsequent contributions to the fund and payment of all subsequent premiums for the fire insurance policy with the $250,000 self-insured retention shall be on the basis of a proration based on the proportionate share of total building values occupied by those county departments in the year the premium and contribution to the fund is due. This proration will of necessity vary from year to year depending on the value of the buildings, additional construction, and changing degree of usage by the county departments involved. The risk manager shall be responsible for calculating these figures. (Added by Ord. 80-103 § 1, Nov. 26, 1980).