I always tip taxi drivers 'over the top' at 40 -50%.
I rarely use taxis but when I do it is because I need one desperately.
The big tip policy ensures that I am remembered, and get one fast next time.
In restaurants I give 20% if the food and service have been good. If it hasn't, I don't tip at all, and tell them why!
I operate that same policy with hotel staff in UK- but on trips abroad I pay up and keep shtum, because I want to get home unscathed
