You should sit it about where you would a normal english saddle. I like the pommel to be just a bit more forward, the front edge just over the shoulder blade.
You fit the pommel by taking the pommel out of the saddle. Put it on the horse about where you'd want it when saddling. The angle of the bottom of the pommel should match up with the horse's shoulders. If the bottom is flatter than the horse by only a little, that's fine as your pad should make up for that. If the bottom is very flat, you need the next narrower pommel. If the bottom is steeper, so the outside edge puts the most pressure on your horse, then you need the next wider pommel.
Barefoot also makes a "soft pommel" for horses that are super flat/wide at the shoulder. This one will mold to the shape of your horse, but has more substance than stuffing the pommel with rags or batting like some have done. Stuffing the pommel gives you a little stability while riding, but allows the saddle to mold to the horse's back. It works best for horses that have no withers and are very, very wide at the shoulders. Horses with any kind of descernable wither should use one of the pommel inserts.