Break it down:
If you go to a massage place, the practitioner is getting only a portion of the fee. Everyone knows that the $ 65 is a point of entry fee. So if you get more than a basic massage ( Happy ending, bj or more ) you're not really "tipping" you should be PAYING this guy for escort services.
If you go see a person privately at his place, then he's getting the entire fee, and tipping is a sliding scale. If you got what you paid for and no added effort to provide exceptional service, the tip is optional. If you got excellent service, a tip, is like a restaurant for excellent service in the range of 20%. If you got more than just a massage, then again you should be paying extra for extras. Same if the session turns to a full-on escort service. You paid for a massage not an ass-fucking. So you're no longer "tipping" you're paying extra for extras. So if the price was not discussed, use your observations of what a good fuck is going to cost you and add that to the price.
Simple.