Someone posted a graphic showing the proper length for the jacket. I don't have a link to that post.
However, the general rule is to have it finished to end so that it does cover your behind, plus about an inch. Then, depending upon your proportions, adjust it as much as a 3/4 inch. So that someone with proportionatly long legs would add half inch to the length. Shorter folks with shorter legs would try to get a shorter coat, giving them longer appearing legs.