These guys beat these shafts to death then blame it on the cuemaker. I'll say 12.8 is safe. more rigid and stable less chance of them losing a set, then slamming the cue and breaking the shaft at the end
12.8 is fair anything smaller is a bunch of bs i make ld shaft, however i'll do 12.5 but if you break the end of the shaft off because of some miscelaneous bs you are on yur own, I stand behind what i make
I recently purchased a used cue lathe that came with a bunch of tips from various manufacturers. To my surprise, many were in the 9-11mm range, none above 13.
For the cues I have reworked or re-tipped, all have been in the 12.25-13.0mm range, with 12.5 being the most common by far.
I personally shoot with a 12.25mm, 30" long CF shaft.