A more interesting metric would be: You are meeting a buddy for dinner at some pizza place at 18:00. You are there punctual for whatever reasons, at what time would you ask yourself whether you would text your buddy? I think in Germany, the first text message might run 18:05 or 18:10 with lose ("I am sitting inside already") kind of a thing.

The haircut example would just mean you tell your friend in advance that you will arrive later.