http://www.brazoscustom.com/magart/0503.htm
More reading:
http://www.brazoscustom.com/Home.htm
So basically... standard angle is 60-65 degrees for real steel, but check for airsoft. In any case, if you're off, you can just do a general cut and you should be able to get the sight to fit with some filing, as long as you get the width to be able to fit in snugly. Secure with some red loctite.