The best way to determine whether you've got the sapwood thickness correct for a replica is to look *directly* at the back of the bow. If you've got the right sapwood thickness AND the correct cross section, you should see a strip of heartwood down both sides of the sapwood. If the sapwood is too thick, it wraps over the fat portion of the cross section, which means you'll not see heartwood. If the sapwood is the correct thickness but the cross section is not round enough, there will be a "corner" where the back meets the side of the bow, and you won't see the heartwood.