However, I only see an area that shows either too much pressure OR rubbing on her right wither. I don't really see a correscponding one on her left side. Odd, huh. Maybe the saddle tree is not straight. I might have it looked at by another saddle person to be sure the tree is straight. It never suffered any trauma that could have broken it? I mean that is not likely. But the marks are not even.
Take a look at her back, from directly behind her and a bit above her to see if her back is even or if the unevenness is in HER body. Make sure she is squared up before you guage her body.