I assume that you're talking about laying out the width profile (back of the bow?) You need to just follow the grain and forget the idea of stright lines. When I'm laying out a bow on a crooked stave, I lay out a centerline first. Measure the width of the stave and find the center at the handle and fades (or you can sometimes follow the crown.) Then do the same thing everywhere the stave changes direction, and connect the dots. You can lay out the width at any point then by measuring out half the width on each side of the centerline. Also remember that the taper may not always be even and straight-if there's a knot or something, you may have to follow the grain out around it to let the bow swell a bit at that point to avoid cutting through the grain. With some practice, you can grab a pencil between your thumb and first finger and let your ring finger run along the side of the stave and act as a guide as you slide your hand down the stave-follows the grain automatically. It's not as hard as it sounds laying out a snaky bow, you just have to fool with it and you'll figure it out. Do away with your notions of straight lines, because they usually don't exist on a stave, and when in doubt, follow the grain.