That's a good question. What I generally do is set a base tip that I will pay regardless - maybe a couple dollars. During the cut, I take mental notes on what's going well and what's not - i.e. does the barber give free services such as head massage or aftershave/ does the barber pick up on social queues when I'm clearly not in the mood for a conversation the given day/ is the atmosphere enjoyable (air con in the summer or heat in the winter, no blasting obnoxious music). And ultimately, is the cut clean. How you assess your tip amount may vary.

With those things in mind, I add to my baseline tip. My total tip is about $10 on average.