I`ve read that the correct contact point of the cushion should be 1 7/16"above the bed.The midpoint of a 2 1/4"ball is 1 1/8".Why wouldn`t the optimal contact point be the exact center of the ball,rather than 5/16 above center?Obviously,the 1 7/16 nose height is correct,but the physics of it escape me. RKC? Thanks.