I have many talents, but math is not one of them, far from it. So, I have difficulty figuring out a 15% tip when I go to restaurants. Dan (my fiancé) taught me a trick of finding 10%, (moving the decimal point a digit) then halving that and adding the 5%. I'm not sure if 20% would be any easier. Aside from that, I find this method of tipping illogical, and rather unjust. For one thing, the quality of service has nothing to do with how much you spend on food and drinks, in fact it can be the opposite.
A couple times, I and/or Dan have gone to restaurants where the waitperson tried to push a product we didn't want. On one occasion, at an Applebee's, a waiter obnoxiously promoted a new appletini every time he came by. (Really are you sure? It's really good!) And it was lunch! Needless to say, he didn't get a very good tip. Tipping is the service industry's equivalent of commissions, but it makes no sense since they are not supposed to be salespeople. I'm fine with it, and these folks need the money, but I've decided a flat amount like $2-5 seems about right, depending on the class of restaurant and the quality of service, including them putting up with my indecisiveness.
Give more at cheaper restaurants, as they need the money the most. And at fancier places, I suspect the valet parking guy needs it more. It also seems arbitrary and ambiguous who you are supposed to tip. It may vary from place to place. Hairdressers, cab drivers, sometimes hotel maids. Basically, tip well any service-person who makes a good effort, is helpful, is patient with inconvenience and probably gets a low wage. And, if the person treating you stiffs them, and you think they deserve more, sneak some dead presidents onto the table. Also, I do not tip people who only serve me at the counter, unless they have to put up with me being difficult, or go out of their way for a special request (hold x, y and replace z)