I cannot become comfort with the US way: Waitresses and bar counter personal are not payed, but work for tips, only. This way I get disenginuous friendliness if not submissiveness by sweated service, and way more "service" than I asked for, which means while sitting in an restaurant eating and adult's talk is being interrupted frequently by somebody annoyingly asking if something is needed while all glasses are still full, and everything had been okay if nobody had asked.
And I don't see why a waitress shall earn more money if she serves me a large steak with a glass of wine, and less for a simple soup and a glass of water while the service she does in both cases is the same.
Most of the times I just want ordinary, common service. Especially when I'm not in a noble restaurant but having just a cup of coffee I simply like to just enjoy my cup of coffee and what I read, neither need nor want some affected bumsucker buzz around me all the time.
In central western Europe tips are a kind of an appreciation, a bonus - if earned. All tagged prices include everything, also the wages for all staff. So if everything was okay, you got what you pay for, then there is no tip. And that is okay. No tip is expected at all.
Most of the times you just round up the check. But that's no real tip. That's just for to bill easier and quicker - even if this brings a few extra bucks into the waiter's purse.
Tip is for anything better than expected, above normal, for extras in service. Then its amount is somewhere between 0.5 to 2 € for small amounts, and 5 to 10€ for larger things, and sometimes 20€ or even more for really extraordinary things, especially in noble restaurants. Or if your wages are above average, and you're not an asshole, or experienced life at lower wages yourself, you give a 0.5...2€ minimum tip every time.
But in general tips are adjusted by feeling, not calculated on a fix percentage base by the bill.
So a waitress may get 30...200€ extra per shift to her wages - or not.
Nobody returns to a restaurant or a bar whith bad service or food. So we see no sense in subtreshold communication about bad service or bad food afterwards. For what? For "I gave it to them"? They will never see me again, so I don't care if they get it or not. Exit talks are pointless.
Btw. when you clearly ordered the check, and it won't come within 20 minutes, it's legal to leave the restaurant without paying. (Watch out! In some countries like e.g. Italy to pay at the table is uncommon, even if sometimes it's possible. In Italy you go to the register and pay there.)
Personally I like the way I know from Italy (there are other countries with this habit, too)
Many of the bars or restaurants have a tip jar beside the cash register. Not only guests may throw their tip into that - nobody takes a look, how much you throw into it, while banknotes stick out, of course - but also after every bill all extra amount above the check's amount is taken out from the register, and put into it - cash.
After the shift the boss distributes that money to all employees, excluding himself. This way all staff, also the guys in the kitchen get a tip. Because they all worked together as a team for the comfort of the guests.