If only a portion of the corporate area of a city is included within a proposed district, and 12 or more registered voters reside within such portion of the city, the formation petition shall be signed by voters residing within such portion equal in number to at least 10 percent of the total number of registered voters in such portion, or 10 percent of the number of voters of such portion voting for all candidates for the office of Governor at the last general election prior to the filing of the petition, whichever is the lesser number. If less than 12 registered voters reside within such portion of the city, the formation petition shall be signed by at least 50 percent of the voters residing in such portion.