When positioning a saddle on a horse, the main thing to look at is the front of the tree in relation to your horse's shoulder blades. For a western saddle, the front edge of the front concho should be about 2" behind the back edge of the horse's shoulder blade. For an English saddle, the front d-ring, where it attaches to the saddle, should be about 2" behind the back edge of the shoulder balde.
The girth should hang straight down from wherever it is on the saddle. Some saddles have forward, mid, or center rigged girths or billets. It all depends on the saddle.
Now, the saddle girth placement may not match up well with your horse's shape. If your horse has a rotund belly or a forward/narrow girth groove, then a saddle with mid or center rigging is not going to work for you. For western saddles, most good saddle shops should be able to move the rigging if it's attached to the tree. If it's in-skirt or dropped rigging, then you'll need to sell the saddle and buy one that works better for your horse.