20% of the bill, including tax. I only round up, never down. If the service or food is exceptionally good, the tip calculation changes accordingly. I seldom tip less for bad service, although I will complain to the server and to the management if the server doesn't take me seriously or offer, in some way, to "make it right." People who take responsibility for their screw ups deserve some encouragement. Everybody messes up, it's what you do about it which says the most.
The one thing that will make me deduct from the tip is the server telling me the problem, whatever it was, was my fault. While I accept responsibility, I don't pay for blame. They can pay me.
I never tip less than $5 for anything, even a glass of water, except for take out.
For take out, I tip less; usually just put the change in the glass if it's not too much -- say between $0.50 and $3. An espresso bar is not take out, even if you get the coffee to go, it's a bar. If the take-out is special food which demands a lot from the kitchen, I might tip more.
I tip the delivery guys for the delivery, not for the food. All of our places are within three minutes, so I tip $5, whether the bill is $10 or $50. I'll tip more for a longer drive or if there's something unusual about the order. The idea that someone's going to spend 15 or 20 minutes taking care of my order for minimum wage is not acceptable.
Unless something very unusual happened, I feel it's in bad taste to call attention to myself with the tip.
If the place is a real favorite, I'll buy the owner a bottle and the staff a case for the holidays. Sushi bars used to cost me a lot, because I bought a bottle of good whiskey for each sushi man, but I only have one sushi-ya and two sushi men to which I currently feel loyalty. We often get gifts from our favorite places, too.
I don't tip more at places we frequent with the expectation that they'll treat us better next time. The tip is not contractual, it's gratuitous (hence, a "gratuity"). I tip what I feel is fair and generous because that's what I expect from myself.