How do you convert backspace to offset?

I thought it was Backspace - (Width/2) but that keeps giving me the wrong answer.

For instance a 18 x 9 with 6.5" Backspace I get:
6.5 - (9/2)
6.5 - 4.5 = 2.0
2.0 x 25.4 =+50.8 mm Offset

Correct answer should be +40 mm according to specs.

If thewheel mounting flange is 2.0" from Centerline why would it not be 2.0" (50.8 mm)Offset?

Edit: Okay I figured it out. Backspace includes rear flange width but the wheel width is measured flange to flange so you have to subtract the rear flange width.

[(BackSpace - Flange Width) - (Rim Width / 2)] x (25.4 conversion to mm)
[(6.5 - 0.5) - (9/2)] x 25.4
(6 - 4.5) x 25.4
1.5 x 25.4
38.1 mm Offset (Im off 1.9 mm because I dont know actual flange width)