I tip the person who delivers my mail around Christmas. I also tip housekeeping crews if I stay at a hotel.
Tipping can be a pain and come off as a scam. I tend to be performance-based. I usually do 10-20%. If you keep my water glass filled and give great service, 20%. If I have to flag you down, it's 10%.
For the OP, 10-15% feels reasonable. Were they really plugged in and giving good feedback? What does that even mean for a trainer? Whatever you do, you'd be setting a standard. It also depends on how generous you are. Service industry people can usually get tips.