Jon, 5/8" might be ok with osage, but if you started there with a piece of yew, juniper or cascara, you would end up with a 20# bow. I am not saying always start out with 2" of thickness. What I am saying is that the thickness at which tillering begins is dependant on the species, variation between individual staves, and also the bow design. One simply cannot state a value that will work for all species (and designs) as the previous poster requested. It is just not possible.