For joint county proceedings, the auditor where the petition is filed shall file a certified copy of the viewers' report with the auditor of each affected county within 20 days after the date of the final order establishing the system. When the final order to establish the drainage project is made, the drainage authority shall determine and order the percentage of the cost of the drainage project to be paid by each affected county. The cost shall be in proportion to the benefits received, unless there is a contrary reason. An auditor of an affected county may petition the drainage authority after the final order is made to determine and order the percentage of costs to be paid by the affected counties. The drainage authority shall hold a hearing five days after giving written notice to the auditor of each affected county. After giving the notice to the auditors of the affected counties, the drainage authority may, at any time that it is necessary, modify an order or make an additional order to allocate the cost among the affected counties.