You must have two conditions set:
you must have X number of bushels stored above Y bushels safety margin.
X is 20/40/60/80/100/etc--the standard sell levels. This is how much gets put on the market.
Y is your reserve.
So, if you set the rule to sell 100 bushels whenever you have 100, then you will need to have 200 bushels before the offer is placed.