the servers receive 18% gratuity on the total of the bill before taxes, and from what i understand, most are pleased w/the number of folks on the dining plan as they are making better tips than before...
we found that for average service, we did not add to the tip unless we ordered something OOP as well (i.e. cocktails) and we tipped on that amount... for above average service, we tossed a few bucks on the table (in addition to any OOP bill amount)... for exceptional service (which we experienced at boatwrights) we left a considerable additional tip because it was well-deserved!
it took a few meals to get over that "cheap" feeling for us as well...