[Sadie]

Look at what an indie charges for what you want, that's what your "tip" should be.
The attendant gets a small cut of the door and for the most part is working for tips.
On average as an indie I'll get tipped $20 on top of my hourly or half hourly rates by the clients who tip, consider the money you pay at the door rent on the room and a tip for your attendant and a convenience charge for having 3-7 girls available 24-7 for you to choose from (and that really IS convenient if you're a last minute kind of guy)

Only exception to this logic is if you're laying with your hands at your sides the entire time, unless she tells you to rest your hand elsewhere, and you smell nice. Than just tipping the difference between what an indie charges and what you paid at the door is great.