The angle of the blade is completely irrelevant as it relates to cutting surface. Blade LENGTH, on the other hand, is exactly what defines it. If a broadhead has a 2" cutting diameter, and has 2 blades, it only stands to reason that each blade accounts for half the cutting diameter, in this case, 1" each.

In the case of the slick trick mag, it has a CD of 1 1/8", but a total cutting area of 2 1/4". Measure tip to tip of two opposing blades, and you'll get 1 1/8"....Think of this symbol....+ Tip to tip of the horizontal line is 1 1/8, and tip to tip of the vertical is 1 1/8.......1 1/8 x 2= 2.25.....