Feel the Stop
As I start the club back by turning my shoulders, I keep turning until I can't turn them any more. That's my signal that my backswing is over, not if I feel like my clubface is square, or that my hands are on my shoulder plane or in any of the over-taught positions that, in my opinion, make it hard to swing free, fast and easy.
If you can't turn back all the way like this, that's okay. You can only do what your body allows. Better to obey it than to force anything.