When I was young, my mom taught me that the rate to tip was $1 for every $7 (in essence, 15%). She was good at math, so this was easy for her to remember.
The first time I went out with a friend to a sit-down restaurant as a preteen, I incorrectly remembered it as 7 cents per dollar! OOPS. So mortified. To the waitress from the Friendly's at the mall in the 1980s - sorry! I knew something was wrong when our tip was like 40 cents apiece....
More recently, as a university student, I was out with my roommate and her 3 friends for a moderate level sit down restaurant. I knew her but had never met those 3 guys before. Well, we had a regular meal with regular service, but at the end, the waitress accidentally gave us the check for the next table only, which had only 2 people! And we were 5 eating meals! The 3 guys got so excited and started to wink and whisper about dropping a $20 on the table to cover the incorrect bill and running out the door! I knew this would come from the waitress's paycheck, so I wasn't going to let that happen. I tried to see if I had enough cash on hand to discreetly leave what I thought was the full amount of our bill, but of course I didn't. So then I did signal the waitress and inform her that the bill was wrong. Those 3 guys gave me the stinkeye all night. Losers! Like that e-hell story, they were all adults going out for dinner, and forget tipping, how could they think of not even paying for their meals?