Ideally, gravity should be your friend. I'm not sure there is a "proper" position...depends on your build, the horse's build, how the saddle is constructed and what you are doing with the horse. But in many saddles, when you sit in them, there is a position that gravity and the shape of the saddle tends to put your leg. If that position is the one you like, then the saddle fits you. If it is one you don't like, then you can shove your leg somewhere else - but now you are putting tension in you leg, and fighting gravity and your saddle. It is possible, but why would you want
that? And I am convinced that in most cases, riding with a relaxed
leg is better than riding with any given leg position.
Our Circle Y will
put your thigh at a forward angle. The shape of the seat has a groove for your thigh. I like a more vertical thigh, and the Circle Y drives me nuts. Our Abetta doesn't seem to have that groove. Add in cloth fenders, and it is much more forgiving for different leg positions. I normally ride my Australian-style saddle with 1" stirrup straps, but there is still a 'sweet spot' where my legs tend to hang relaxed. For that saddle, the sweet spot has my heels closer to my hips than I like. Oh well. Since I believe a relaxed leg is better than a leg 'position', I accept it and ride the saddle. If I could afford a custom made saddle, I'd have the stirrup bars placed further forward.
My daughter's leg is in the groove of our Circle Y Mojave saddle. I prefer a more vertical thigh, but I would feel more comfortable with my heel further forward. But that is pretty much where my leg hangs on my saddle, and it is close enough.