Producer means any dairy farmer who produces milk approved by a duly constituted regulatory agency for fluid consumption and who delivers pool milk as specified in section 20.14 of this Part to a pool plant, a pool unit, a plant specified in section 20.28(f)(2) which is partial pool plant, or a partial pool unit whose pool designation was canceled for failure to meet the requirements specified in section 20.26(a), except that it shall not include any such dairy farmer delivering to such partial pool plant or partial pool unit unless at least 50 percent of such dairy farmer's milk delivered to such plant or unit is pool milk pursuant to section 20.14 of this Part. Each dairy farmer delivering milk to a partial pool plant or a partial pool unit shall be considered to have delivered pool milk for such dairy farmer's proportionate share of total milk delivered by dairy farmers to such plant or unit.