The amount of the assessment for necessary expenses shall not, however, exceed the following:
(a) In the case of producers, 212 percent of the gross dollar volume of sales of the commodity which is affected by all such producers regulated by such marketing order.
(b) In the case of processors, distributors, or other handlers, 212 percent of the gross dollar volume of purchases of the commodity which is affected by the marketing order from producers or of the gross dollar volume of sales of the commodity which is affected by the marketing order and handled by all such processors, distributors, or other handlers that are regulated by such marketing order during the marketing season during which such marketing order is effective.