A proper saddle fit begins with a basic understanding if your horse's anatomy.
This is a picture of a horse's skeleton. Notice the shoulder and rib configuration.
This picture shows the shoulder bone or scapula of a horse.
This picture shows a saddle tree, sitting in the correct position on the horses back. Note it is sitting, back behind the shoulder bone so the horse has free movement of his shoulders. It is also sitting level on his back.
This picture shows a saddle placed properly on the horse. Note the position of the scapula bone and the corner of the saddle tree. It doesn't matter the horses conformation, the saddle is always placed behind the shoulder bone.