Thanks for saying this @Tactile Daddy.
I always take into account the time and effort that is put into a massage, with or without any extras. As a matter of fact I never go expecting extras, I just go with the expectation of having a solid massage and whatever happens beyond that happens.
Instead of trying to figure out an amount to tip, I always come prepared to give $100/60 min, $150/90 min, and $200/120 min no matter what. I think this is reasonable for the service they're delivering, and it's still less than what alot of independent MT's ask for on MassuerFinder or RentMassuer. I've never had a complaint or received any attitude about my tip, if anything they express alot of gratitude and I tend to get excellent services whenever I see them again.