Folks asking about the tax need to check with a US tax accountant (there are some in HK). What I know comes from talking to US citizens. I believe the 85K also includes accommodation if supplied.
Some people I've meet have been ticked off about the double taxation. Some want to pay US tax only. Some want to pay HK tax only. Some want HK tax to be more like US tax, a million pages of tax regulations but with more possible deductions.
If you're not earning the 85+K, you're fine. Otherwise start your search for an accountant and see if you can get the cost included in your expat package.