It depends on the service I receive. If the server/busperson clears my empties without having to be asked, the tip goes up from a 10% base, Akso, how many dirty dishes did I generate? Did I make many trips to the buffet, or only once through the line? Did I use soup, salad, dinner and dessert plates, as well as soda glass and coffee cup?
The more work for the server/busperson, the higher the tip...........
Did the server/busperson offer to get me a refill on my beverage? or bring clean napkins? or offer to carry a plate for me to the table?
These actions merot rewards................
BUT>>> If the dirty plates accumulate, and gthe server/busperson has to be summoned to clear, the tip goes down.
If I never saw a server/busperson at my table from start to finish of the meal, then the maximum I'd leave for final clearing of the table is $2 per person.
Here in Connecticut many municipalities have laws requiring patrons to use clean plates for every trip to the buffet. Lazy servers/buspersons have a habit of clearing plates but not drinking glasses. If my lips have touched the glass, then it should not be put against the soda dispenser. Take my dirty plates, but leave the empty glass to be reused, the tip goes down.
I live in the US so that may distort the response but I don't think I'd do 15%, probably 10%. I mean you're standing in line, serving yourself, I assume carrying your food to the table. The type of service that requires is set up so they can serve the masses so I think $3.50 (CA or US) is plenty.