If they prefer, customers can send in some basic measurements, and a video. The purpose of the latter is to observe the rider's style, and whether it hinders the horse. It's important how they ride, how the horse moves with and without the saddle. An uneven rider will play havoc with the shape of the saddle. In comparison, an uneven rider will cause the saddle padding to compress on one side or the other.
Balance is another consideration.
The old rule stating that the saddle has to be level across the top from the pommel to the cantle no longer applies, as saddle manufacturers offer many different heights of cantles, depending on the discipline. Again, the channel should never be narrower that those magic three fingers. The ligaments, like tendons, have very poor blood circulation. There should be no pressure on the spine. Schleese warns that saddles, over the course of riding, will collapse slightly, so channel width should be checked periodically.