My backyard range requires that for 40 yard practice I stand on a slope (towards the target) of about 20 degrees, ie my back foot is about 4 to 5 inches above my front. This seems to affect my form to the point that I develop some back pain after about 10 shots. The discomfort seems to locate in my lower back on my draw (left) side. Also my accuracy suffers as well. At 20 yards my groups are inside 4 inches, at 30 they increase to about 5 inches, but at 40 suddenly the circle goes to 12 inches. Any advice?