It depends on the method of knapping you intend to use and the width of the points you intend to make. That rock is way too big for my saw, or I would cut it for you. I use a lot of pressure flaking on slabs, especially if I have made them really thin, but if I intend to use indirect percussion for the primary thinning, I make them a little thicker Here is my general intention:
Pressure flaking - 4 or 5 to 1. That means I need to know how wide I am going to cut the pieces from the big slabs and set the thickness accordingly.
Indirect percussion - 3 or 4 to 1 but no thinner than 5/16 inch for even the narrow ones. Indirect percussion is really playing with high risk for breakage until you learn what you can do and what you can't do. Trial and error teach that. One thing I do on all indirect percussion of thin stuff is hold the stone/slab/preform tightly under a board of wood when hitting to drive a flake off the bottom of the stone. The board prevents any whipping motion in the stone and prevents a lot of breakage.
WA
