Hmm, there are a few different ways people seem to use leg & seat aids. Some are almost opposite to others.
Perhaps if what you're doing causes him to yield his hindquarters away, you're either 'supporting' too much with your inside leg, or you have your outside leg too far back & he's yielding in the opposite direction?
I use my legs to 'push' the horse in the right direction. Therefore, to turn on the forehand, I look & turn my upper body in the direction I want. This pretty much automatically makes my legs, seat & reins go where I want them. My outside leg on about the girth, or even more forward when first teaching, my weight shifted to the outside, rein on the neck, inside(loose) rein out from the horse a little, inside leg & pelvis out a little, to kind of open the way.
If I want a hq yield, my leg is further back to 'push' the horse's rump over. I turn to focus towards that quarter and my weight is rocked forward, to free up his hq.
All this is pretty exaggerated when I'm teaching a horse, but I refine it as we go, so the cues end up being very subtle in the end.