If it were me, and I have never done Birch Bark, but I have used other backings. I would get it tillered first before the backing. Get the brace tiller spot on perfect, then get it tillered out some distance your comfortable with, 15 inches or so, maybe even 20, and get the tiller looking good at that point. Then put your bb or rawhide on. I am assuming you are concerned it will snap without the backing if you go out to full draw?