The board of supervisors of each affected county shall determine the rate of each of the assessments referred to in Section 75370 by deducting 15 percent for anticipated delinquencies from the total assessed value of the land in the district within the county, as it appears on the assessment roll of the county, and then dividing the sum reported by the board, as required to be raised in the county, by the remainder of such total assessed value. If a fraction of a cent occurs on a valuation of one hundred dollars ($100), it shall be taken as a full cent.