Chim,surfran,
I think surfran is right if what you're trying to do is create a frame of angle iron so that the folded edge of the angle iron is furthest from the inside of the frame. The angle you're trying to find is what would be notched out if you were to bend the angle iron by 90 deg and then have the sides of the angle just meet at the folded corner. If you had a chop saw, you wouldn't have to do it the way you're doing, trying to calculate the angle to cut with a hack saw. Anyway, the angle is the arccotangent of 1.414 (sq root of 2) or 35 deg. Like surfran says, just lay out 35 deg and saw along the mark and it should fit fine. Next time, get a chop saw. Good luck.