There are two thoughts on this
1. If it's drilled and matched perfect so be it
2. Temperature, specifically barometric pressure can affect thumb size. Arthritis, capillary swelling due to use among other things, cause swelling. Sweat from lights, etc., etc., etc..So going slightly bigger on the thumb and using some tape can ensure an exact fit.
I have seen a ball of Chris Barnes' and he had 6 pieces of tape in it.
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