Its very simple. 20% OR MORE to a server that does their job and does it well. 18% to a server that does what they are supposed to do, but doesn’t go that extra mile. 15% to servers that delivers your food and never come back to check on you. ….and that’s it, it shouldn’t ever go lower than 15%. In my opinion, there is no such thing as not tipping. No matter how bad the service is. The server will get what they deserve by management if they are that bad, that shouldn’t be up to you to decide you aren’t a server critic, you don’t really know.