I do not leave a tip based on the percentage of the total bill. The reason for this is if the food sucked its not the servers fault(unless it comes to me cold), so why should it be based on the bill. I base the tip I leave on service. I pretty much use a $10/hour rate excluding the $2.xx/hour rate they get paid. The most I am ever in a restaurant is an hour and that is pushing it. If I am with a group of friends and get charged a gratuity forget me leaving anything extra. To me that means your a shitty server and are resorting to any means necessary to get a tip. It usually pretty sad because looking at the gratuity charge it is usually 50-60% less than what I would have left. All I ask of a server is to make sure my glass does not hit bottom. I have a lack of saliva problem due to radiation treatments back in 2002 and nothing is worse than running out of water to wash the food down.

EDIT: I usually get pretty bad service since I look like a po' bitch most of the time. IN these cases I still tip on service since you should not base a book by its cover.