(1) As soon as all of the assessments have been equalized and adjusted as aforesaid, the commission, by resolution, shall charge each lot, tract, and parcel of real estate in the district with an assessment equal to the benefit estimated in the assessment list to accrue to the lot, tract, or parcel of land.
(2) The commission shall apportion the assessment into annual installments so as to realize annually only that sum of money as will be necessary to meet the expenses incurred in each year for interest and a sinking fund to pay the bonded indebtedness with twenty percent (20%) for contingencies.