The easiest way to determine the amount to remove would be to insert a .45 ACP round into a chamber, and measure the the head protrusion from the face of the cylinder. Use several rounds and several chambers and take the minimum. Then measure the thickness of the .45 AR case rim, which should be about 0.09" Again, measure several rims and use the thickest measurement. Subtract the fomer from the latter, and that should be the amount required to machine down the cylinder face.