The Council shall set the initial assessment at no greater than one tenth of 1 cent per gallon of odorized propane. Thereafter, annual assessments shall be sufficient to cover the costs of the plans and programs developed by the Council. The assessment shall not be greater than one-half cent per gallon of odorized propane, unless approved by a majority of those voting in a referendum in both the producer and the retail marketer class. In no case may the assessment be raised by more than one tenth of 1 cent per gallon of odorized propane annually.