18's with 275s on a Duster

What i like about backspace is that it can be measured with a tape measure and a straight edge. And i dont even have to know how wide the wheel is.
I have several wide wheels with very wide tires, that makes it hard to tell if they are 12" 13" or 14"wide. Without knowing the width, how do you calculate the offset?
If i look at my wheel, and one has 5 1/2 bs, and the other has 3 1/2, i know which one will fit on my car.
Tell me you think that wheel is a 14", you think, and you think the offset is +50mm, or whatever, i have no clue whether that will fit.