Such ordinance shall set forth the total amount of such new assessment and shall provide in what manner it shall be apportioned among the various lots and parcels of land included within the assessment district in said ordinance defined, giving due credit to each and every lot and parcel of land entitled thereto for all payments on the previous assessment for the improvement in question that shall appear from the city records to have been made on account of such lot or parcel, and for any part of such previous assessment, which, though unpaid, is a valid lien against such lot or parcel.