heh the math is maddening (not hard, just maddening).
the darn things grow in 3s (1 plant -> 3 seeds -> ... cycles) and stack in lots of 50. They are consumed in 3s, but the food stacks in 10s.
so 10 onion soup is 30 onions, but you would want to grow 25 or 27 maybe, or grow a stack of 50 to store. Or whatever. No matter what you want to do, it never seems to work out evenly!