I would use the upper "clamp" as the edge but that still leaves you with little adjustment. Assuming the bike is a good fit for you and not the reason for a strange saddle fit cheating is possible, but if you are not a light weight a rail can fail. Knowing lawyers help design the safety margins/factors on every thing these days from what I have seen of saddle failures the worst outcome would be a long ride standing or walking. A replacement non-weight weenie saddle with stronger rails might help some as well.