Hi derek ...
I just had a look at the movie and I find that despite the blurriness, I can see what fingers I'm using. But, maybe you can't see the movie (Mac?) so:
The G/B: my index is handling the 5th string bass note; my pinkie is on the first string; my ring finger on the 2nd. Same applies to the Bb ... I just move the index down one fret for that, leaving the pinkie/ring fingers where they are.
It's usually just plain old logic that tells you where to put fingers. There are several possibilities for any shape, and after a while you just automatically know what to do ... rather your
hand knows -- like tying your shoe lace, or sharpening a pencil. I don't really know of hard fast rule though, and if there is one, break it as often as you can. The more ways you learn to do the same thing, the better off you'll be.
I'm not sure what the 'play the B in 7th position, G#7 in 4th position means' ... that's not one of mine ... but the E shape barre chord for B is at the 7th fret, and the G# E shape barre chord is at the 4th fret ... so I would think it's someone's way of saying 'play those chords as E shape barre chords'.
I hope I helped!
