15% is concidered the bare minimum for acceptable service. 18% is the more the accepted standard now, especially in business settings. Great service can ben 20-25% if you are an exceptional tipper.
I use my own scale based more on guests served. We have kids, and a lot of times they don't get their own meals, so I tip by the guest. 4 of us at a place where they basically give you refills and move plates (buffets or counter order w/servers) $1 per person. At a standard fare sit down -- Chili's, Friday's, Don Pablos, ect, $2 per person. If at a fancier place, maybe a steak place or upper scale italian or something where meals are at least $15 each, $3 per person. At something grand, like a WDW sit down meal where the meal is $20 or more, $4 per person. This usually keeps the tips anywhere from 15-35%. But it is a tip based on the ammount of work they do. If we add dessert, it's another 50 cents from each of us.
So a $30 meal gets an $8 tip. A $50 meal would get a $12 tip. If the meal costs $80 or so at say the Melting Pot - $16 tip. If we get dessert there -- $20. Sometimes if I do order a steak at the Chilis and the bill does come to $50, usually I'll still only tip that $2 per person or $8. Yeah, it's a lower %, but how often does a table w/2 adults and 2 kids under 6 give you $8 as a Chili's waitperson? They would have done the EXACT same work for me if we would have split 2 burgers and had a check of $14.
Yes, I have worked as a server for several years, and I do find this to be a personally logical way to tip. It may not be what's generally done, but I do believe that the person is working for a desired ammount for a table for a time period. If I hold up a "4 top" I should tip what a "4 top" would tip. $2 per person at a standard fare sit down. It is very generous when a lunch crowd can be automatic gratuity and every person gives their 94 cents and calls it a day.