Use the TDC mark on the fan, and a timing light with an adjustable dial.
If you don't have one of those, then you will need to calculate how far from the TDC mark you need to make another mark on the fan. (Since the stock mark is 27 deg BTDC, you can divide the linear measurement by 27 and multiply by whatever the timing is supposed to be and put the mark that far away from the TDC mark.)
...Ooooooorrrrr, you can just wing that sucker!
--DD