I use my back muscles like a draw stop too. Some time ago, I used that to determine where my face anchor should be. I use Arne's rotational draw to the point where I’ve reached maximum rotation, and at this point my string hand is about 1/2” from my face, which I believe Arne also does. I bring my string hand into my face and that's where my anchor is.
Interestingly, this doesn't necessarily determine an exact draw length. As I said, over the years as I have shrunk, and even though I use the same rotation and same anchor, my draw length has decreased. Also, when I use a heavier bow, my draw length decreases, not much, but a little. I think when I use a heavier bow, the extra weight pushes my joints together more than a lightweight bow does, and reduces my draw length that way.