In the U.S. the etiquette is that you tip 20% as long as you ate your food. They literally pay waiters less than minimum wage because tips are expected to make up for it. Unless your server fucks up apocalyptically you really shouldn't tip less than 20%. I don't know what the rule is in other countries. I know in Germany you just round up to the nearest dollar.

Yeah I personally live in Denmark and tips aren't really expected. But that is linked to the salary being much higher for waiters here indeed.