I think losthuntr is right on. You also must decide what you're trying to accomplish by measuring spread. I mean, are you using B/C as your model or the "I got-a 30" buck" as your model? A lot of guys claim 30" bucks but they don't tell you that they measured the widest outside spread, regardless of its location or geometry. In the end, it's all up to you but it makes a huge difference when field scoring live deer.
I also would caution you on using spread as the determination, that is, if you're trying to find a good scoring buck spread is only a very small portion of the total. If your after spread only, be careful about judging with the ears as they tend to vary by a few inches. Also, when judging by ears pay very close attention to the spread distance, inside, of the main beam. A buck can look much wider than he turns out to be if the main beams are not real wide but the rest of the antlers are, or the main beam curves or narrows up. In fact he can look like he's 2" outside the ears on each side but turn out to be only 25" wide becasue the inside spread of the main beam is narrow.
Finalshot