Just for the sake of discussion. Some say their fingers are to short, fat etc for a 13 mm shaft. They may prefer a 12.75, 12.5, 12.2 mm shaft for this reason. The difference from 13mm to 12mm is of course is 1mm or appx .040 thousands of an inch. At 12.5mm its .020 thousands.
What i read is your index finger won't expand 1mm, give or take? 1/16 of an inch is 1 1/2 mm or .062 thousands. Think about that, its a very small amount.
IMO the real reason in most cases isn't index finger length. Many play with an open bridge so finger size is moot. IMO you just prefer to aim with something smaller, you think it deflects less or adds more spin etc is the real reason. With exceptions for some with arthritis etc.
My index finger from the first bend at the hand/palm is 3" long. I think I have small hands and short fingers. I could easily play with a 14mm shaft. I don't, I prefer 13 + with my short fingers.
So unless you have very small hands, I don't buy the, my fingers are to short, fat etc reason. How long is your index finger? Just curious.
Rod