To levy and collect taxes as provided in section 235.160, the board shall in each year determine the amount of money necessary to be raised by taxation, and shall fix a rate of levy which, when levied upon every dollar of the taxable tangible property within the district as shown by the last completed assessment, and with other revenues, will raise the amount required by the district annually to supply funds for paying the expenses of organization and the costs of the district, which rate of levy shall not exceed forty cents on the one hundred dollars assessed valuation.