Here's a tipping question from another angle. You're on a business trip and can pay for a meal up to (or something approximating Seamlessweb for delivery), say, $100(inc. tax and tip). This is a pretty hard ceiling (accounting will chew you out if you hit $101).
How much can one in good faith spend on food, before tax and tip? For some reason, this feels different me than the question of "how much to tip on a bill of $x."
Bear in mind that quality of service has no bearing here, because your total payment probably set from the moment you order. Under no circumstances can you go above $100.
For reference, about $83 plus 20% is $100, $86 for 15, and $80 for 25.