Personally I'd recommend putting a little more than 15% away each year.... you will probably get hit with provisional tax (i.e. double tax)year on year in addition to the 15%, and while the previous year's provisional offsets the next to some degree, I've found a more realistic round figure of about 20% of total taxable earnings is a bit more on the money, as you always seem to earn a little more each year.
If you earn bonuses, profit share, etc, that will probably be billed as well. Tax on housing only seems to apply to pilots, thanks to a delightful deal CX did with the IRD a few years back... most other expats don't have it assessed as taxable income from what I've heard of other expat contracts. As always, check the details with your employer first, as things do seem to vary considerably between companies.
Honk.