Additional pro rata assessments may be made when any special assessment roll proves insufficient to pay for the improvement for which it was levied and the expenses incidental thereto, or insufficient to pay the principal and interest on bonds issued in anticipation of that assessment roll; provided, that the additional pro rata assessment shall not exceed 20% of the assessment as originally confirmed, unless a meeting of the Council be held to review the additional assessment, for which meeting notices shall be published and mailed as provided in the case of review of the original special assessment roll.