Unless you get a thumb slug mold, each ball will vary slightly depending on how much you have the PSO open up the thumb hole, and also where he opens it up. Typically I start with a size and the driller sand it down, then I tell him to open up each side of my thumb to get the feel that is ok for me. I also take a piece of tape and immediately put it in and try it before leaving to be sure it is OK.
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