Are you talking about bamboo or Hickory backed bows? For a Bamboo reflex deflex bow that I typically make, The belly slat is about a 7/16 or 1/2" with a 1/8 or 3/16 taper from handle to tip, that will get me a 60-65 pounder. But their are many factors involved, meaning amount of reflex,deflex, recurve, length, draw weight and so on and soforth. I guess It is kind of hard to answer your question without knowing what exactly you are after. For someone who has never made a backed bow I would suggest watching Dean Torges' video about bamboo backed bows, this will get you started. Also probably not a good idea to make a tri-lam bow on your first try. Hope this helps some.