(c)that in fixing the rate, at which the tax is to be imposed, regard shall be had to the principle that the total net proceeds of the tax, together with the estimated income from payments for water or lighting as the case may be, supplied from the works under special contract or otherwise, shall not exceed the amount required for making, extending or maintaining the water-supply or lighting system, as the case may be, together with an amount sufficient to, meet the proportionate share of the cost of supervision and collection and the repayment of and, payment of interest on any loan incurred in connection with any such supply or system;