Do you have an actual sit bone width measurement?

The nice thing to have is outside of one bone to outside of the other, and then add a cm or 2 to each end. (my sits are about 170mm, add a cm to each end gives me 190 or so, I ride a B67 and everything fits!)

The back half of the Brooks has a fairly wide (maybe 1.5cm?) metal plate/frame on the underside that the leather is anchored to, and you don't want to end up sitting on the metal frame. You want to be planting your sit bones on the suspended leather to the INSIDE of the frame.

(in other words, the outer edge of cantle is a safety margin and gives you wiggle room, but isn't for spending your sitting time on.)

LisaSH had a B17-S that she was sitting on the metal frame too much. (pardon my grammar) The saddle is still nice and all, but you don't get the indentations into the leather that form to your bones or get the fab-o comfort of being on the suspended leather. She could tell you how that went. (she and I have wide sit bones, so don't go thinking you can't fit a B17!!! It depends on size.)

If you order from www.wallbike.com you'll have 6 months to try out the saddle. That will give you plenty of time to find out where your sit bones land and if you have the right saddle.