Twenty percent? What!?! No waaaay.
Having said that, usually we leave about twenty percent because the math is easy. That assumes perfect service. If the food itself is lousy that is a matter for management, not the server.
My oldest daughter was a waitress during her college years and is the final arbiter (when she's visiting) over tipping. When she isn't here then we wing it.
Coffee cup empty = halftip
Everything else is negotiable.
The more people, the bigger the tip percentage. My table of 7-8 adults and 3 grandkids (ages 1-4) will take a lot longer to turn over than a table of 2-4. So, the server is making less per body on top of the less than minimum wage. So we tend to tip larger and be more tolerant. Although, if you let my coffee cup go dry....... bad juju, terrible karma, and upside down mojo.
I have on occasion left zero tip. I also left a note with the manager on the way out explaining in ugly detail why there was no tip. On occasion I have asked the waitress to bring the manager with her so I could thank her in his/her presence.
I'm Phil -- 40 something heterosexual white male, fairly self sufficient, great with my kids, wed 29 years to the same woman, and I firmly believe that ones actions have logical consequences. How much more out the box can you get nowadays? -- MSgt of Marines (ret)