Signatures of 15% of the qualified voters of the City shall be required to propose legislation by initiative petition and the signatures of 10% of the qualified voters of the City shall be required to order a referendum on any legislation. The total number of qualified voters shall be determined as of the date of the last general election. The date the first signature on an initiative petition is affixed shall be no later than 90 days after the issuance of the ballot title on the measure, and the initiative petition shall be filed with the City Recorder for certification and approval no later than 180 days after the date the first signature on the petition has been affixed or at the end of 100 days from the effective date of the ordinance codified in this chapter, whichever is later.