I was rewatching one of Bigtruck's videos on youtube that included Eric Durbin and James Davis Jr. Durbin played pretty amazing. But in one game he got hooked on the five ball and he was using his cue to make angle calculations on the kick. However, you can see him twisting the cue to make a chalk mark on the railing felt. The blue chalk on blue Simonis cloth makes for a subtle marking, but it is clear enough to make out. I've never seen a professional do this. Is it legal?
Here's the vid: https://youtu.be/1ufZyEv4RSE?t=32m42s
Here's the vid: https://youtu.be/1ufZyEv4RSE?t=32m42s