The farm marketing quota for any crop of cotton shall be the actual production of the acreage planted to cotton on the farm less the farm marketing excess. The farm marketing excess shall be the normal production of that acreage planted to cotton on the farm which is in excess of the farm acreage allotment: Provided, That such farm marketing excess shall not be larger than the amount by which the actual production of cotton on the farm exceeds the normal production of the farm acreage allotment, if the producer establishes such actual production to the satisfaction of the Secretary.