Locate his PAP, draw arc from PAP 3 3/8" crossing from top thru the general area of the ring finger hole, down thru to just left of the center of the grip. (This may be the difficult part to picture because a PAP can vary from 6" to 3". If the PAP were 6", then the pin's leverage position would be well outside the ring finger hole. If the PAP were 3", the pin's leverage position would be under the bridge.)
From the center of the grip, draw a line to intercept the PAP's arc line drawn above, close to the point which would look like 10:30 on a clock (this is the mirror image of 1:30 for a righty). Where this line meets the PAP arc is where one would place the pin. A 3" pin-CG distance is kind of lengthy; unless the bowler has a large span, the CG will wind up below the actual center of the grip.
P-F-----F- ---> F = finger holes, P = pin
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-----C---- ----> center of grip
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-----T---- ----> T = thumb hole
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"Those who do not remember history are doomed to repeat it."