I had a couple of Kielwood shafts built and was undecided on the diameter. I already had acquired one that’s
12.85mm. So I went with a popular size I was already using, 12.75mm, but for the other KW shaft I got 12.6mm.
Gosh, 12.6 -12.65mm seems to be ideal for a shaft that’s neither too skinny nor too thick. As I earlier wrote, I played
with 13 mm shafts and have no issues with that size. My cues have a 13mm shaft & 12.75mm but 12.6mm is terrific.