Personally I always feel obligated to tip even when the service isn't good. I've always been conflicted though. On the one hand it's like "well you didn't give me a good service so do I really need to tip you?" On the other, waiters and waitresses rely on tips to make a living so I would feel bad if I didn't leave a tip. So I decided to do 10-15% (10% if it's really bad but I would still like to leave a small tip) if the service is bad/decent and 20-25% if it's good service.
I think in this case you kinda cheaped out. lol It's not a big deal I just thought it was funny that you assumed stuff and only tipped him 50 cents. I would have gotten change for the $5 bill and maybe leave him at least a couple bucks.