In every marketing agreement and order the Commission, with the advice of the commodity board, shall prescribe the per unit rate of such assessment, and such rate may be at the full amount of, or at any lesser amount than the amount hereinabove limited. The rate may be altered or amended but only upon compliance with the procedural requirements of this chapter for amendment of marketing agreements or orders. In every such marketing agreement or order the Commission and the commodity board shall base their determination of such rate upon the volume and price of sales of affected units, or units which would have been affected units, had the agreement or order been in effect during a period which the Commission and the commodity board shall determine to be a representative period. The per unit rate of assessment prescribed in any such agreement, order, or amendment shall for all purposes and times be deemed to be within the limits of assessment above provided until such time as such agreement or order is amended as to such rate.