How do you tip your tattoo artist?

It’s been 4 years since my last tattoo. Yet my “old friend” works at a place called Big Guns tattoo. He’s doing one for me next week. $120 an hour. It’s a smaller tattoo, about 6” by 4” on my shoulder. Colored. How should I tip him? I know it’ll be absolutely beautiful and I know it’ll be perfection. How do you tip your artist? What’s standard? What’s expected?