Driving Saddle Placement
Driving saddles generally have trees just like riding saddles. It's very important that the saddle not be placed too far forward. It will affect the movement of the shoulder. I find most drivers that have come from a riding background really want to put the saddle too far forward.
The saddle should sit a solid hand behind the shoulder blade:
The girth should hang straight down from the saddle and be about 4 fingers behind the elbow so it does not affect the movement of the elbow; the elbow needs to move without being knocked by the girth.
The girth need not be tight as well. Especially because driving saddles are not carrying weight like a riding saddle, they do not need to be tightened down. I always say a hand between the barrel and girth is good.
Hope this helps with understanding how saddle placement should be!!