I'm no drilling expert, but I recently converted to ITs as my thumb decided to have a mind of its own and started changing sizes a LOT. I had been using oval slugs and just finished putting them in 7 balls....then within 2 weeks my thumb shrunk 2 sizes, so my pro shop guy suggested it was time to make the move. Made sense at that point. What we did was drill 1 ball and make 1 thumb, to make sure I was going to like it. Once we did that, I had him actually make me a 2nd one the same size so I could be lazy and NOT swap out my spare ball every time....plus I would also have a built in backup. Well I ended up having him make me another one 1 size up (with a matching 2nd for the spare ball). Those kept me going for a while, while he put the anchors in all my balls. So I had those for a little while, and noticed my smallest one was getting loose, so had him make me 1, one size down. No 2nd on that one yet, but now I have 3 different sizes. It is so nice when you feel your thumb swell (or shrink) and you can just swap thumbs and get the same feel each time!! Good luck