The assessor shall prepare a copy of the assessment roll, to be known as the city tax roll, and upon receiving the certification of the several amounts to be raised, the assessor shall spread upon said tax roll the several amounts determined by the council to be charged, assessed, or reassessed against persons or property. He shall also spread thereon the amounts of the general ad valorem city tax according to and in proportion to the several valuations set forth in said assessment roll. To avoid fractions in computation of any tax roll, the assessor may add to the amount of the several taxes to be raised not more than the amount prescribed by law. Any excess created thereby on any tax roll shall belong to the city.