How much do you think an average waitress deserves as a tip?

I used to be a waitress before I was a nurse. I averaged about 15/hr at Chili’s. I’m curious to see what people think SHOULD be the average percentage to tip. Don’t say 20% just because it is now 20%. And don’t say less than 10% just because you don’t believe in tips. Choose what you think is an average tip for your everyday waitress. Personally, I believe in 15%. I know that they are trying to push 20%, but I know waitresses who make a very decent living with no college education. Serving is hard but it does not require the knowledge and time that other careers do. I was happy to make 15/hr but not happy to find out that there are servers who make more than I do now, which would also be more than firefighters, police officers, EMTs, nurse aids, medical assistants, teachers, and etc. Also, how do you feel about tipping off of percentage? Do you think it’s fair to tip based off of food price when the servers could be working equally as hard? Thanks for the input :)