In creating the special fund, the city or town legislative body shall have due regard to the cost of operation and maintenance of the system as constructed or added to, and to any proportion or part of the revenue previously pledged as a fund for the payment of bonds, warrants and other indebtedness. It shall not set aside into the special fund a greater amount or proportion of the revenue and proceeds than in its judgment will be available over and above the cost of maintenance and operation and the amount or proportion of the revenue so previously pledged.