At least 90 days before any interest date of the bonds, the treasurer of the district shall certify to the county treasurer of each affected county an estimate of the amount of money and the percentage of the assessment together with the interest thereon, or only of the interest, necessary to pay interest and principal or the interest maturing on the interest date after crediting thereon the funds in the treasury applicable to the payment thereof, to be collected by such county treasurer, and shall add thereto 15 percent of such aggregate sum to cover possible delinquencies. In determining the amount of the funds in the treasury applicable to such payment and to be so
credited, the treasurer of the district shall not take into account the bond reserve fund, if any, applicable thereto unless he is directed to do so by the board in accordance with the terms upon which the bond reserve fund was established.