Your link doesn't go to a specific saddle, so I don't know whether you are looking for english or western. The above website is a good explanation of how to do a wither tracing. With this information, a saddler can help you find a saddle.
I also would take the first measurement and trace it onto card stock, cut it out so you have a physical representation of the wither. Go to the local tack store if you are lucky enough to have one nearby, and use this mobile withers to put under the saddles and get an idea if they follow the contour of the horse.