As mentioned before, feel where the saddle naturally wants to rest into place (without a pad or girth). It looks about right. A couple other rules of thumb are for the tree points to be about 3 finger-widths behind the shoulder-blade. Often, the edge of the shoulder-blade is where the mane ends at the base of the withers. It looks like your tree points are about 3 fingers (or 2 inches) behind that area. Another is for the girth to be about 4 finger-widths (or was it 4 inches?) behind the elbow. Again, that looks about right. I agree with Stella that the saddles balance point looks a little far back. The cantle is a little higher than the pommel though, so it doesn't really seem to be all too narrow (looking at it from the front might help see that better). I wonder if the design of your saddle simply has the balance point a little far back?
Your noseband does seems very tight. You're not trying to clamp your horse's face shut, just keeping him from gaping wide open.