The amount of state aid to be allocated to a county pursuant to this part shall be determined by the department in the manner the department determines is appropriate. The department shall review the county’s statement of authorized expenditures actually incurred and if satisfied shall provide state aid in an amount not to exceed 3/4 of the county’s estimated authorized expenditures for the past calendar year. If the county’s authorized expenditures actually incurred for the past calendar year exceed the county’s estimated authorized expenditures for that calendar year, the department, if it considers it to be in the best interests of the state and adequate funds have been appropriated by the legislature for state aid to counties, may provide state aid in excess of 3/4 of the county’s estimated authorized expenditures for that calendar year, but not in excess of 3/4 of the county’s authorized expenditures actually incurred. If the amount appropriated by the legislature for state aid to counties is insufficient to pay the full amount to which the counties are entitled, the department shall reduce the allocations proportionate to the shortfall of revenue among all state and local programs for which waterways account money was appropriated.