I never fully understood that. To me, no matter what the angle is of your legs, the guitar is always going to rest on the same spot, at the same height. Since your upper thigh isn't really moving up or down even if the stool is.

I'm assuming you are just supposed to sit with your feet flat on the ground and the guitar resting on your leg (how I've done it for 14 years). But like you mentioned, this means the strap isn't really carrying the weight. The point of it I think is to make the strap hold the guitar at that same spot when you STAND. Ideally, you shouldn't be slinging the guitar way down low since it requires more of a bend in your wrist.

Maybe just sit like you are now, but tighten the strap until you feel it carrying some weight? Doesn't really look "cool" standing with a guitar up to your chest though.