AFAIK, as long as you are in Taiwan for 183 days of the tax year (Jan 1~Dec. 31), you will be taxed at the normal rate as opposed to the 18 or 20% “foreigner” rate. Therefore, you would get a refund of the excess amount withheld (withheld at the “foreigner” rate) when you file your taxes. So, simply (but accurately!) count the number of days on a calendar.