Maybe start at 10%? It really depends on your satisfaction with the service. If they rock it out and give you a world class experience you can go higher.

My wife is a RMT and that is total asshattery to not tip. Your RMT is lucky to get 45-50% of your service fee, and unlike a dentist doesn’t have 3-4 Dental Assistance underneath them doing all the menial dental cleaning each day.