Im probably alone on this but if they're standard dont really keep my drinks full and dont seem to friendly then 10% if they are friendly but service is standard i do 15% If they are just fantastic i will do 20% but thats rare to find
another thing i dont understand is this lets say order 2 burgers and it cost me 20 buck.. the waiter has to make the same amount of trips to and from the kitchen as if i say i order 2 steaks at 60 bucks.. so because i want to spend more on myself he is entitled to a 300% increase in tip for the same amount of work?
Lots of restaurants have their servers tip out the support staff (bartenders, food runners, busboys) based on % of sales. So, if you hold to what you mentioned above based on what you perceive to be the same amount of work, the model breaks.
I never looked it up, but it's no different than commission based off sales in any field - sell more, make more.
I generally throw 20% down for decent service.