(d)that in fixing the rate, at which the tax is to be imposed, regard shall be had to the principles that the total net proceeds of the tax, together with the estimated income form 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 re-payment of land payment of interest on any loan incurred in connection with any such supply of system;