I re-checked the tipping "suggestions" which appear in the various weekly guides one finds in your hotel room. And even with (what I consider) extravagant suggestions in some cases, only states 15% for taxis and does not mention a minimum.
But in any event, since a tip is a voluntary thing (otherwise it would be part of the price), the driver should NOT have made demands and made derogatory slurs at me.
Why are you still worrying about this? 95% of the people on this thread (and in my estimation, probably 95% of the people on the planet) agree that what you tipped was fine.
I would have shook my head at the driver's audacity and forgotten about it by the time I hit the escalator.
The world is filled with jerks. Don't let them ruin your day.