My experience regarding stock dimensions on a Darne pretty much mirror Bills, with the exception of cast. Almost all semi-pistol Darne stocks I have encountered had significant cast off. 3/8" seems to be the norm with a few examples pushing 1/2". Straight stocked Darnes, I've owned 3...have all been cast neutral.
Another note...the Darne is the strongest double shotgun action made, it's only competitor to that title is probably Hoenigs Rotary Round.