One way to determine if you have the optimal centershot is to do the walkback method. Put up a small target at the top of the target butt and sight your bow in to hit the x on it. Then drop a plumb bob line down so that it lines up with the X on the target . This will give you a straight line up and down from the center line. Then step back to 30 yards and shoot an arrow still using your 20 yard sight . Then step back to 40 and shoot an arrow at the target still using your 20 yard sight. You go back as far as you can and still be hitting on the target butt. Then you look where the arrows landed in relation to the left or right of the plumb bob line. If the arrows track to the left of the string as you go back move your rest in small increments ,1/32 of an inch,to the right . Then go back to 20 yards and resight your bow to hit dead center again and proceed to do the walkback shots till your arrows are hitting in the same plane. You may have to do this several times till you get them hitting on the same plane but when they do you will have found the optimal center shot for the bow and arrow combination you are using.