HK$100 is fairly standard for a $700-$800 HG who
gives you good service, and correspondingly less
for a $300-$400 walkup.
I used to feel bad about not giving a tip, but
these days, if I'm not happy about the service,
I just don't. OTOH, with a girl who tries really
hard and gives good GFE, don't be shy to give
more than $100.
BTW, make sure you have enough change
(bring mixed bills) so you aren't forced to
give more than you intended.
