I use a right angle electric drill equipped with a rough grit sanding disk to grind down the concave back, grinding off the marks of the Forstner drill bit, and grinding down to the lever where the center point of the bit has penetrated the wood. When that central hole just disappears under your sanding disc you know you are at the thickness that you chose when you set up your drill press to hog out the wood with the Forstner. You can also use that rough disc to straighten up the perimeter of the now concave back. Be sure to not grind off too much. Keep removing the accumulating sawdust so you can see what you are doing. There are several wood turning calipers that you can use to test how thick the back is becoming. The basic rule" when you cannot see the little holes anymore you are at the proper thickness."