So, I tend to see 15-20% as a reasonable norm. I am rather bothered by the increasing "nudge" on tipping...I've noticed an upwards push on the "suggested tips" from 15/18/20 to 18/20/22 to 20/22/25. I actually ran into some taxis where the prompt was 20/25/30, which I found insulting.
Also, some terminals only seem to allow you to do a custom percentage (I usually try to "round off" with my tips) rather than a custom dollar amount. An issue with that in at least one taxi resulted in a zero tip not because I wanted to stiff the driver but because the system acted up.
As a rule, I will admit, I'm to the point that with car services I generally don't tip anymore. There are exceptions (e.g. "I need to make my train" and the driver makes it happen) but they are, well, exceptions.
I think I generally draw the line at 18/20/22...anything "pushing" beyond that is an insult to the customer. And, per above, any "service charge" which is levied lands as a tip in my book.