I don't think I've ever tipped less than 15% as an adult. Bad service included. Everyone has an off day, an off shift, an off hour. And everyone has to put food on the table. Yeah it sucks that you ran into it, but fuck it. That's how the world works sometimes.
Generally I tip:
20 - 30% on good service
15% otherwise
$2 for beer, $3 for mixed drink (lol like I'm ordering a mixed drink tho)
Hair and shit...guys you need to tip. Like for real. I tip $5 at the place I get my eyebrows threaded and $8 on my hair. Job is too important not to be rewarded.