Right. But how much? I'm not from the US, but I know that the expectation is to tip. And I do. However, every time I leave some place I have this vague feeling of unease and guilt - technically I could have afforded to tip more. Was it enough? Did I offend them by accidentally calculating 15% instead of 20% ? Is the norm actually 30% in that place?

It'd just be so much easier if it was already included in the price.