In every marketing agreement and order the director shall prescribe the rate of such assessment. Such assessment shall be expressed as a stated amount of money per unit or as a percentage of the receipt price at the first point of sale. Such rate may be at the full amount of, or at any lesser amount than the amount hereinabove limited. Such rate may be altered or amended from time to time, but only upon compliance with the procedural requirements of this chapter. In every such marketing agreement, order and amendment the director shall base his or her 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 director determines to be a representative period. The 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.