Hereafter in any case where a public improvement is about to be made by any county, city, borough, town, township, or municipal district, or division, in connection with which the corporate authorities intend to assess the cost of such improvement, or a part thereof, against the owners of property benefited by the improvement, the said corporate authorities shall have power to cause to be prepared preliminary estimates of the amount of benefits which may be assessed against the owners of such property and for which liens may be legally filed under existing laws; and, in any proceedings for the incurring or increasing of indebtedness for the borrowing of moneys for the payment of the cost of such improvement, such estimates of assessable benefits, when signed and verified by the affidavit of the engineer that they are in his opinion fair estimates of the amounts of benefits which may be lawfully assessed, shall be considered deductible assets in such proceedings, in calculating the percentage of the assessed valuation of taxable property representing the total indebtedness of the county, city, borough, town, township, or municipal district, or division, in the same manner and with like effect as if liens had been actually filed for such benefits.