When a district is in more than one county, the total annual estimate shall be divided by the board in proportion to the value of the land of the district in each county. This value shall be determined from the equalized values of the last assessment rolls of such counties, as revised by the boards of supervisors. When such division of the estimate has been made, the board shall furnish the board of supervisors and auditor of each affected county a written statement of the part of the estimate apportioned to that county.