The point at which a servo centers is determined by the placement of the body of the potentiometer inside the servo. If you absolutely must change a center, open the servo case and rotate the pot body by the amount you want to change the centering. Just make sure you dog or glue it down afterwards or it could rotate and change center position in use. When a servo is manufactured, the minor differences in tolerance build up makes an exact duplication in all servoes next to impossible.