The bars of the saddle should not exceed the last true rib. it's ok if the skirt does as long as it doesn't rub the hips. I'm curious as to what the tree is made of. Take a stiff coat hanger and make a form and fit it over his back about two fingers behind his shoulder blade. Be sure it is perpendicular. The google horsesaddleshop and download and print their free templates. Use the shape of the wire to compare with the templates to find what tree you need. One half the template should fit the horse, the other half the saddle. Works great if you transfer them to cardboard.