Re: Cheek piece adjustment/eye relief/sight picture questions
The benefit of a cheek piece is if you have a scope mounted up high because of a large objective. You can raise the comb to get your eye in line with the scope. If you've got a 40mm scope with low rings, you may not need any added height with the cheek piece. Move it up and down and see what's most comfortable. Close your eyes when you get behind the rifle. After you're settled in, open them. Ideally, you should be in line with the line of sight through the scope.
If you plan on shooting mostly prone, adjust your scope for eye relief for this position. For most people, the scope should be a little farther forward for prone than is necessary for offhand or kneeling. Get comfortable behind the rifle, and with the rings loosened, move the scope back and forth until you get the complete sight picture. That is, no shadowing around the margins of the sight picture. If you get shadowing on the bottom of your sight picture, you may need to raise the cheek piece to bring your head up a bit and vice versa.