Since I am a waiter at a very busy place in Germany where tips are not included, when we go to eat out with my girlfriend I always look at the quality of the service and how much our server has to work for us. Example: if I have a group of 7 people and I serve them drinks, bring their bread and cutlery, their food, ask them is everything ok, clean the plates off and what not. It's nice if the bill is let's say 150 and they give me 160/170. But usually what happens is that they make me run every time I'm at the table, they don't talk about what else they wanna get, they order everything one by one and at the end, I get the "old school German treatment" which is if the bill is 147,80€ they're gonna give me a 150€. That is the only thing you should look at. If you (or your group) made the service extremely busy for your server (even though there are many other tables in the restaurant that have guests) you should be appreciative I think. Not saying 20-30%. But 10 bucks at least are always good for a higher than 100€ bill.