Tip on the cost of what you were served. Not the what you were charged. If you BFF bartender comps a drink, include that in your tip, and do that for all service workers for everything you're served.
Yeah, I feel like the options for including comped stuff in the tip are either add 20-25% of the value of whatever of the comped thing costs or add 100% of the value of the comped thing to the tip. You don’t…leave it out entirely, that’s uncivilized