can you post a pic of the stave? i establish the center line of the bow first by following the crown on the back side of the stave. so, if you imagine looking at the back of the bow, you would draw the center line along the highest part of the crown of the bow- the ridgeline, if you will. project this line through the bow to the belly and mark the center line on the belly. (assuming that you have the belly side of the stave flattened),. now that you have the center line on the belly, decide on your width, and how it will taper. starting at the center (handle) mark your handle width by measuring out from the snakey center line. then move out towards the tips, marking the sides of the bow equidistant from the snakey centerline at whatever width you have decided on. . leave it plenty wide at first. as you shave down the sides, you will get a feel of where the grain runs. if your taper is good, then your tool will cut smoothly from handle to tip, not snagging on any growth rings. mark the center of the bow, then mark every 6 inches on both limbs out towards the tips. mow check the width at each mark to ensure that corresponding marks are equally wide.....
right, and the straight edge line on the edge comes in later as a fool proof tillering technique for rollercoaster limbs, as mspink implies, but first, lets get the sides laid out