The assessment rate may vary from district to district and from crop to crop based on the degree of vulnerability to damage from curly top virus experienced by those crops in those districts. The rates may also vary in accordance with the protection afforded to those crops in those districts. Any rate or rates established shall be set by regulation. The assessments shall be in total amount sufficient to reimburse the secretary, and whenever feasible, shall not exceed 65 percent of the expenditure by the department in carrying out the beet leafhopper control program.