Not sure with yew, as I don't work with it, but it's my understanding that it can go thicker and narrower than most anything else. With most woods, you will end up somewhere in the half inch range, so your pretty safe starting rough at 5/8-11/16 and slowly tapering out to the tips. If you go pyramid style, you will remove wood evenly, no thickness taper, to get within target weight range, and adjust the tiller with the width taper. With other styles, you will keep the front profile set, and taper the thickness out to the outer limbs. Paddle bows are a bit weird to tiller; they taper in thickness to mid-limb and then not so much in the outer limbs. In all, I would start at 11/16 for anything but yew, and would wait for someone who works with it more (Del, Gordon...) to chime in on what they typically see for thickness with yew. The major difference between other woods is not so much in the thickness, but in the width of the limbs. You can even make a bow out of white pine, the limbs will still be about a half inch thick, but will be around 3 inches wide.