My record is 3 out of at least a dozen attempts. I found that those lop sided preforms that are thicker down one edge are less likely to snap in half. Flake from the thin side, trying to keep the thickness where the shank of the hook will be. Keep an eye on that centerline along your edge. If you try to flake from the the top side of it or sometimes even too close to it, you'll halve it.