Tipping
The percentage is the easy part. Every genuinely confusing spa tipping question is about which number to take the percentage of, and this handles all four of them.
The discounted, membership or promotional price at the desk.
What the same treatment costs at the rack rate. Same as above if there is no discount.
Common US practice at spas is 18 to 20 percent, the same range as a restaurant. Nothing obliges you to any figure.
The convention is the menu price. A therapist does the same 80 minutes of work whether you booked it at full price, on a Groupon or with a gift card somebody else bought.
Many resort and hotel spas add 18 to 22 percent automatically. Check the bill before adding anything.
For a couples massage or a group booking, each guest normally has their own therapist.
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A therapist at a commission spa is usually paid a percentage of the service, and on a discounted booking that percentage is already smaller. The 80 minutes of work is identical. Tipping on the rack rate is the convention across the industry for exactly that reason, and it is the single most common way people accidentally underpay.
Tipping is a custom, not a rule, and it is a US-specific one. Independent therapists who own their practice often do not expect a tip, some medical spas do not accept them at all, and automatic service charges sometimes go to the house rather than the person who worked on you. When in doubt, ask the front desk. Nobody there finds the question awkward.