A saddle maker told me the gullet is measured from "concho to concho" on the tree. The problem is, by the time the maker pulls the fleece lining up into the gullet, that measurement changes. (He also said the way the material is pulled into the gullet can have a tremendous affect on the saddle fit -- that's why fitting bare trees may not guarantee a good fit when the saddle is finished).
He also said that the gullet does not a semi-QH or full QH bars saddle make. It's the angle of the bars that determine that. Put your hands together, palms facing, fingers straight up. Then pull your palms away, leaving your fingers together. You can change the angle of your palms without pulling your fingers apart. Your fingers are like the gullet (in this case, the gullet is 0" but you get the idea). It's very often the angle of the bars that make or break the fit of a saddle, not the gullet.
There are no standard measurements for gullets for saddles. It used to be that 6 was QH, 6 1/2" was semi QH, etc., but with more manufacturers making more and more saddles every day, those numbers don't mean much anymore.
There's a nice article at: http://www.horsesaddleshop.com/howshoulifit.html
You could give the buyers the finished measurement from concho to concho unless you want to remove the underskirting... Adding the leather/skirt underneath can subtract 1/2" to 1" from the actual tree gullet width. If you measure from concho to concho (squishing down the fleece well) and add a half inch, you'll be close to the gullet width of the tree. As far as the bar angles, I think QH bars are about 90 degrees, but full QH bars are more.