@BCDC - I would agree totally with what he is saying about putting the thumb further towards the body of the guitar.
I spent weeks trying to squeeze the barre down, with my thumb either behind the index, or pointing towards the headstock. All I got was a sore thumb joint. Justin doesn't seem to mention thumb position in his barre lessons, but I'm sure a few videos on YouTube show a "pinch" between index and thumb.
Moving my thumb behind finger 2 was a revelation. I'm not sure whether I got the idea from a YouTube video, or possibly it was mentioned in the very long "dreaded F Chord" thread. Almost immediately I was able to form good chords all the way up to F on my electric, with very little pressure needed. In effect you are forming a lever, with the force coming from the arm, not your thumb.
The other benefit is (for me) - with thumb either behind finger 1 or (worse) pointing up towards the headstock, my fingers 2,3,4 tended to collapse towards my index, which as Justin says, is bad. With thumb behind finger 2, this does not happen.
Acoustic is harder of course but I'm getting there. Now all I need to do is learn to change to other chords at a decent speed!