Don't think about 'choosing' or 'teaching yourself' a back angle. Like Adam said, The back angle just
happens as the result of how everything else is set up. One general 'rule of thumb' as a starting point for the knees is to be about an inch forward of the toes, but considering how far you're sending your knees forward, I'd just thinking of holding them back and getting to the right position earlier.
It will help to have a coach, but if you can't get to one, I'd recommend getting the the book and watching the Art of Manliness videos on the Squat (Youtube). Also, check out
this stickie on videoing form. It's hard to see a lot of what's going on because it's being reflected in the mirror and the equipment is between you and the viewing frame. A rear-quarter view not reflected in the mirror will probably be a lot better.