I wouldn't overthink it. There are no fixed guidelines to time in room or services provided. The attendants are actually reasonably well paid union jobs. The tip is just a way of telling them that you appreciate their friendly attitude and general helpfulness. It is very different than wait staff that really need that 15-20%.
If your attendant is reasonably cheerful and helpful, a $20 as you step off is much appreciated. I don't think I've ever tipped more except if someone provided an unusual special service. One time long ago I developed a bad cold/sore throat on a long ride, and the attendant brought me meals in my room, lots of hot water for tea, and checked in on me every few hours. I wasn't bad enough to get off the train but I wasn't in any condition to leave my room. I gave him $50.
OTOH, if you get a real grump or someone you never ever see (which happens) a simple "Thank you" as you step off is fine.