> I’m surprised that you are 50+ and are saying 15-20% “has always” been the standard for sit down full service. I am 40+ and distinctly remember when it was 10%.

I'm 40+, and when I was in elementary school, pretty much every source I encountered was consistent with 15% as the norm, 20% as a reward for exceptional service, 10% as a signal for tolerable but substandard service, and anything lower as a signal of “abysmal service and I'm only tipping to let you know that I did not forget about the tip to avoid ambiguity.”