Pics are always nice... But anyways to me the problem your raising is the least of it because if you're dealing with metal and your gate post is 10 units long but your post only goes into the ground 4 units deep, you might end up with the gate shaking loose simply because you don't have enough counterbalance going into the ground. I don't know what the formulas are involved but I imagine they're not too complicated. Re your other question pouring concrete into the upright is a decent idea to strengthen everything up.