With the surface being a compound angle, there are 2 angles you need to find. The first angle is easy to find, the second one is a little tricky. How I do it is this:
I have a sine plate templete I created since that's wha he machinists use. I tip the part up on the first angle, then tip it to the second angle. This will make the work face normal to the tool/drill. It's harder for me to explain than to show, but this might get you in the right direction.