The butterfat differential for the adjustment of prices as specified in this Part shall be plus or minus for each one tenth of one percent of butterfat above or below three and five-tenths percent an amount computed as follows: round to nearest one-tenth cent, 0.138 times the current month's butter price less 0.0028 times the preceding month's average pay price per hundredweight, at test, for manufacturing grade milk, in Minnesota and Wisconsin using the "base month" series adjusted pursuant to section 20.51 (a) through (e) of this Part, as reported by the United States Department of Agriculture. The butter price means the simple average for the month of the Chicago Mercantile Exchange, Grade A butter price as reported by the United States Department of Agriculture.