In the segregation and separation of the gross revenues into the several funds in § 6-21-440 stated, the governing body may prescribe a reasonable excess amount to be placed in the bond and interest redemption fund from time to time during the earlier years of maturities of such bonds so as thereby to produce and provide a cushion fund to meet any possible deficiencies therein in maturities of future years. If such excess amounts are thus provided provision may be made in the ordinance for the scaling down of such amounts in future years if the surplus so provided in the earlier years may then be found to have been accumulated and to be available.