I should just straighten this thread out a bit:
Things may have changed but with Dragonair the arrangement with the tax office was that you would pay an extra 10% on the 15% tax you pay on your salary for the housing allowance. That works out as an annual tax amount of 16.5% of your gross salary (10% of 15% = 1.5%). It didn't matter how much you actually received as housing allowance. Things may have changed though ...