Where are you located? 15% for average service is definitely customary at sit-down restaurants in the United States.
I'll tip 30% at like an Ihop when my bill is 11 bucks because "total tips" is what matters.
Taxi drivers get $1 no matter the fare. The gall of that industry to even ask...
And even the slightest infraction of protocol and I leave 0.
I used to overtip all the time. I got absolutely nothing of value for doing it, so forget it.