In my country, which does have a tipping culture, the norm is to give a 10% tip in restaurants as a default, for competent service, decent food, etc. If the service is worse than normal (rude server, cold food, huge wait times, mistaken orders etc) you'd live a lower tip, possibly none at all. If the service is great, you might leave a higher tip - though 20% would be considered huge, 12-15% might be quite normal for very good service.