For below-average service I tip 10-15%. But if the service is really lousy, I will tip $0.25 exctly, just to make it clear I didn't siomply forget -- it's an intentional disrespect, given in reciprocity.
I used to do this, but have since taken a different approach. If you get the manager and complain calmly and in specific detail, you will almost always have some or all of your bill taken care of, the manager or another server will complete the service of your meal with special attention paid to your every request, and the sub-par server will definitely learn a lesson that night. Stiffing the waiter or waitress is a sort of oblique passive aggressive gesture that rarely has the effect you think it does. Often, the server will just think you are cheap and/or a dick. I have worked in that business in almost every capacity, from banquet service to buss boy to barback to bar tender to delivery driver to host to server to prep cook to short order to cashier to kitchen manager, and unless you get very specific and express your concern to their bosses, as far as the server is concerned, your 25¢ tip is just evidence that you're a lousy prick, period. They will
never consider that they may have done something wrong or failed to earn your money unless you make it very clear why and how they fell short. Even then, they probably have some excuse. Trust me, if you get bad service, get the manager and calmly explain why you are dissatisfied, he'll be grateful, you'll get your bill reduced or paid in full, the server will be be reprimanded, and the service at the restaurant will improve. It is the best thing for everyone.