In creating such special fund or funds the board of directors of the district shall have due regard for the cost of the operation and maintenance of the district system required by the district to furnish said irrigation water, domestic water, electric power, drainage, or sewer service, as the case may be, and shall not set aside into such special fund a greater amount or proportion of the revenue of such service or services, than, in its judgment, will be available over and above such cost of maintenance and operation and the amount or proportion, if any, of the revenue previously pledged to such special fund or funds.