klep -- Draw a line on the ball from the middle of the thumb hole through the middle of the finger bridge.  This is the grip center line.  Now draw a line connecting the front of the two finger holes.  Measure from where this line intersects the grip center line to the front of the thumb hole, excluding any bevel.  Divide this distance by two and mark where that distance falls on the grip center line, measuring from either the front of the thumb hole or the line at the fingers.  This is your grip center.  (Other drillers:  Yes, I know this is not exactly how the rules define the grip center, but it's a lot easier to explain and is within a small fraction of an inch of the official method.  And for any use klep will have, it's just as accurate.)  --  JohnP