All funding bonds shall be serial in character and approximately an even proportion of the total amount of each issue shall be payable annually. The amount of the principal due in each annual payment need not be exactly the same but with respect to each installment excepting the last may be made to differ not more than five hundred dollars ($500) from the amount obtained by dividing the total of the principal amount due under the bonds, by the number of installments. The last installment shall be for the balance of the total principal amount not provided to be paid in the previous installments.