I used to double the tax until this thread. Problem is I travel a lot for work in different areas with different taxes and sometimes undertip without realizing it. I’m now using the double the first digit (it’s so simple to think that’s just a quick way to get 20% roughly, but never thought of it).
It’s like starting to say hi to the co worker from another department on your floor each time you see him in the hallway and then all of a sudden ignoring him.