The quick answer for 1/4 cast is move the hole 1/4 off center. Ive recently done this too (stock an Underhammer). The raw plank I used was Cherry, 8/8 wide. I put a centerline around the middle, drew the stock contour on the side, and then marked the drawbolt elevation from the side, using the actual drawbolt in the action to ensure alignment. And then transferred it across the butt. I wanted a fairly wide buttplate, and wasnt worried about placing a patchbox so as not too interfere with the drawbolt, this making it quite easy I think. The head of the draw bolt needed a 1/2 recess, so I added a 1/4 for material outside the hole, and doing this ended up with 1/2 cast.. setting a bevel square off this angled centerline, I transferred a similar angle to the union of butt to action. Did this so the draw bolt wouldnt pull the butt back to square. Finished, the butt is about 1 7/8 and has about 3/16 thickness where the hole is. To drill the hole I welded an extension (1/4 mild steel round bar) onto a 9/32 drill bit, and used an breast drill with a buddy to help with alignment while drilling.. hope this helps .. Kerry