I don't think I've ever left nothing, probably haven't left less than 15% since I was a dumb kid. I worked as a waiter in my early 20s and it made an impression on me how somebody tipping well could make a lousy day better, beyond just the $ - just the idea that there are good people out there (which was necessary when dealing with so many !@#$s as will happen in a service job.) 20% for me is usual minimum, I usually am eating with kids who can make a mess and be a bit loud, when I'm with them it's usually 30%. If it's a cheap meal it's also usually a higher percentage (if it's a 10$ lunch I'll leave 5$.)