Proceedings for a conversion must be initiated by petition filed under § 12-308 of this subtitle. The petition shall: define generally the proposed conversion; request that proceedings for the proposed conversion be initiated under this subtitle; and describe the proposed conversion district by: specifying its exterior boundaries; providing a description of the proposed conversion district according to an official or recorded map; or referring to a plat or map filed with the petition that indicates the proposed conversion district by boundary line. A petition for proceedings for a conversion must be signed by not less than 20 landowners within the proposed conversion district whose property constitutes more than 80% of the assessed value of property within the proposed conversion district. For purposes of this subsection, the assessed value of the property shall be determined by the last assessment roll used by the county or municipal corporation.