You can make do with a 2" thick plank for both builds, but it doesn't leave much room for error in the lay out. The bolster on a flint lock plate is generally around .300" thick - Some Davis plates are thicker. So add 2X bolster thickness to your barrel diameter at the breech, and a little more for insurance...le voila! The cheek piece on many rifles is not thicker that that, but if you have a butt plate that is over 2" wide, take that into consideration too.