We know we can’t take the riders weight beyond the 18th thoracic vertebrae back into the lumbar.
A short back in a horse is not an insurmountable problem unless we have a taller, larger rider. In this case, the deep seated saddle is really not an option; we have to go for a flatter tree or a more "open seated" saddle.
In a dressage saddle we can often use a saddle with a scalloped front. This gives us just an extra bit of back by getting the points a bit closer to the back of the scapula. Putting the saddle further forward is not an option.