Twenty percent of the entire bill, regardless of the amount of the bill, whether over or under $30, is correct. Fifteen percent is becoming less common these days. Leaving a 50 cent tip is beyond rude! If the food or service is bad, speak to a manager--don't stiff the wait person. Anyway, the waiter isn't responsible for the food being bad OR how long you have to wait to be seated or get your food! I'm not sure you understand how restaurants work. Your waiter can't control how many people come to the restaurant or how backed up the kitchen is. The only thing the waiter can control is how polite he is, whether he refills your drink promptly and things like that. You are punishing him for things out of his control.
Another thing you may not be aware of is that many restaurants practice tip-sharing. Each waiter has to share a certain percentage of the tip he is supposed to get. The waiter shares the tip with the busboy, bartender and hostess; he gets no choice in the matter, either. He has to share the tip even if he didn't get a tip! So if you only leave him 50 cents or tip 10%, he still has to share the tip you SHOULD have left him. The restaurant will calculate the amount of his tip share based on the total of all the tickets of his tables that night. The restaurant will assume he gets a tip of 15% at least per table. Out of the 15% per table he supposedly gets, he has to share about 3-5% of that. So not tipping him COSTS the waiter money. This means he PAYS for the privilege of waiting on you and having you leave him 2 quarters! Don't do that to someone.
If you are unhappy with how long you waited for your food or to be seated, speak to the manager. Your waiter will happily get him for you.
Used to be a waitress.