Based on my experience, your barrel channel as viewed from the butt end and above, when laid out on your plank, will run diagonal, muzzle to the right and breech toward the left and butt plate back to the extreme right for cast off. Now your cheek piece will allow a nice taper, wider at the rear and tapering narrower toward the wrist. This layout achieves as much cheek piece extension as possible with a thinner plank and cast off. At least this is my approach.