In Berlin, a tip is not calculated on a percentage of the total. On the one hand, one German teacher told me he would leave a tip of 1 euro on a cappuccino, say, that cost 2.50. But on the other hand if your bill is 9 euros, say, a 1-euro tip is also fine. The way it works is, they tell you the tab, and then you announce what you want to pay (if you'll be getting change). Maybe the bill is 10.50. You say, as it might be, "Zwölf" (twelve) as you hand over 15 euros, and get 3 euros change.
So OK, I think I understand the system, but I never feel I have a firm grasp on what counts as under-/over-tipping. Bourgeois to the core, I just want to glide through life making people feel good in social situations where they anticipate some kind of financial recognition of their contribution. What happens is, I come up with a proposed payment whose tip element would (I know) be grossly insulting in New York*, and I consistently follow the scheme that I think is roughly right, and I keep going to my favourite bar/cafés, and after a while the staff are giving me hugs and offering me free drinks. It's charming but terrifying.
But now my battery is about to die, so I must go.
* For those who have never been to NY: $1 is not a tip.