If it was really bad, I would not tip and probably file a polite complaint. If it was mediocre 10% (or round it to nearest dollar close to the 10% range)
Good 15%
Excellent 20%
There is no excuse for bad service at a restaurant. You pay good money to be waited on and for your food as well. We all have our bad days, sure. But it's simply not professional to bring your personal life to your work place, in any setting. If one can't keep it together as a server, hopefully they can learn before getting out into the real world.
A tip reflects your satisfaction with the service you received from your, hence, server. A good server is there without imposing. if nothing else, at least polite but should be somewhat friendly, they shouldn't make you feel uncomfortable or bad about asking for a refilllet's say, even if they are busy. A great server checks to make sure the order is correct before taking it from the kitchen, fills your drinks just before or after receiving your meal, acknowledges everyone at the table, repeats your orders back to you, and asks you if you need anything.
**If it's take-out, and a place a visit frequently, even if I AM the one picking it up, I always tip a couple dollars. They remember that and it's funny how your food is always that much more excellent and cared for!
**If me and my girls and guys are going out for drinks and we plan on staying at one place for a while, we usually always give them a big tip right away...once again, they pay close attention to your group and your drinks are always plentiful and sometimes a little stronger *wink*