There was a waiter working in a restaurant in Miami back in the 60's I believe. One night, after one of his performances, Jackie Gleason came in with his entourage. Probably 10-12 people. In those days a tip of 10% was acceptable, and meals weren't near as expensive as today. After the meal was finished, Mr. Gleason asked of the waiter, "That was very good service. What is the largest tip you ever received?" The waiter humbly responded, "I was given $500 one night." Not wanting to be outdone by someone's tip, Mr. Gleason gave the waiter a $1000 tip and asked him, "Who gave you the $500?" And the waiter said, "You did Sir."
So, I guess what I'm saying is give what you think is appropriate within your budget, then put the thought behind you.