I've made a few walnut bows, had several break on the tillering board. Made a hickory backed one that was my favorite bow for a year, 50lbs at 28, 68" ntn. It broke the next spring, probably because it was inadequately sealed and dried out over the winter. I have another unbacked one now 45 lbs, nice shooter. I would recommend 68 inches min. And backing is helpful, but not necessary, most of mine that broke, brok in compression, on the belly, chrystalling on the handle taper or from runoff grain in the working part of the limb. If your can find a stave or board with sapwood, a sapwood back, heartwood belly bow looks and works great, my two best have been like that. A pyramid style works well. Good luck.