Here's a good, brief article that sums this up pretty well: http://www.grubstreet.com/2019/04/how-to-tip.html

Until I read that, I honestly didn't know that people tip on top of the tax. I was firmly in the "tip 18 percent, before tax" camp and assumed I was leaving a good tip by doing so. I'll admit that I was a bit outraged after reading the article because it made me feel like a cheapskate over something that had never even occurred to me before.

What other tipping faux pas am I unknowingly making?

I believe it doesn’t. I go to expensive restaurants and bars mostly where waiters and bartenders get way overpaid due to tipping. If it has any impact, it will help smaller waiters/bartenders that do not work in busy places and suffer from their employers not paying them a normal wage.