Outside of a spa this is my philosophy as well. To freelance providers I let them know what I am looking for and they can name their price. If it meets expectations or goes above I’ll usually add another $20 or so but nothing dramatically over the agreed price.
In a legit spa setting 20-25% assuming great service.
NYC style spas I think it’s customary to leave upwards of 100% as the masseur generally only makes tips and the cost of the massage is generally very low.