the position of the stirrup bar, (the place where the fender wraps around a bar that is a part of the actual tree) has everything to do with how long a leg, and how well under you you can keep it. Too far forward, and you cannot avoid a chair seat, and will find posting really hard to do.
I have found that Crates saddles seem to have good balance, that allows for pretty effortless posting.
Also, Dave Genadak makes saddles that are designed to keep the rider with their leg straight downunder them. In fact, many people who are used to typical western saddles, will find his saddles too "vertical". but once you get used to it, stop bracing into the stirrup and use your core and your seat, they are really wonderful saddles.