I generally tip 20% and the easiest way for me to figure it out is $2 for every $10 on the bill (rounded up). Unless service is bad and then I'll drop it down to 10% or so, depending. My friend insists that you do not tip on the total bill, as then you are tipping on tax as well, but I think this is kind of a cheap way out of it, and it's usually not much different.