The fitter who sent it noted that the horse needed a narrow or medium-narrow tree. To my eye - and according to the templates we use - the horse was on the wider side of medium. I sat for a few moments and compared the different templates to the tracing, trying to see how the fitter had come up with medium-narrow to narrow when I was seeing a generous medium. And after a little thought, I figured it out.
Here's the angle I measured to determine tree width:
Here's the angle the other fitter was using:
Here's the difference:
The original fitter was measuring the width too high - too close to the spine - and basing the tree width on the atrophied muscle. Obviously, a saddle that fit based on that criteria would have been too narrow, and would have made the atrophy worse. The assessment I made was based on the muscle that ought to be there (and that would be there with the help of a properly-fitting saddle), with an eye toward getting the frame of the saddle correct and "filling in the dips" with a modified panel - in this case, a wither gusset and a K panel to increase the bearing surface down the mare's quite prominent wither. We ordered a saddle with a "medium +" width - wider than a medium but not quite a medium-wide - because the owner wanted to use a sheepskin half pad for a little extra cushion, and to make up some of the width. The saddle fit the mare really well, and it came back to me about 8 weeks later for its first flocking adjustment. At that point, the owner no longer needed to use the sheepskin half pad to make the saddle fit well. And - happy ending - about 6 months after that, the mare had developed so much muscle that we had to send the saddle out to have the tree widened. The mare's going great guns, and the owner is thrilled.