Howdy, Kayakfisher, welcome aboard. How long, and how wide are the limbs? It's hard to figure a thickness in advance, as each piece of wood will vary. A good general layout thickness that's usually worked for me with heavy woods like osage, locust and hickory (stiff-handled flatbows) is about 5/8" at the fades, 1/2" at midlimb, 3/8" or so at the tips. This usually leaves you with a stave that's extremely stiff, but will bend a little when you start floor-tillering. From there on out, it's just tiller until it's the way you want it.