On or before June 1 of each year, or, if necessary in connection with notification of the due date of the initial installment only at any appropriate time, the chairman of the drainage board shall notify each public corporation to which costs are apportioned as provided in this chapter of the installment of assessment and interest thereon coming due before the next June 1. On or before the due date of an installment each public corporation shall pay to the county treasurer the full amount, together with interest accruing to the due date. Failure to notify a public corporation shall not excuse it from making payment of the installment and interest. In determining the amounts due from public corporations assessed in the aggregate, the chairman shall calculate the total state equalized valuation of each within the area served by the drain and shall assign to each public corporation as its annual installment the same percentage of the total aggregate installment next falling due as the state equalized valuation of the public corporation within the area served by the drain bears to the total state equalized valuation within the area served by the drain. In making those calculations, the chairman shall use the state equalized valuation for the latest year in which the valuations are finalized for the public corporations so assessed, even though the valuations may no longer be current. If noncurrent valuations are used, the drainage board may determine to adjust later payments when valuations are finalized if the board in its sole discretion shall determine the adjustment to be necessary to properly apportion costs between public corporations so assessed.