If you were using osage you could probably go down to 60-62 inches with a good piece of wood. With the hackberry,depending on how good of piece it is, I would not go any less than 64inches myself. Keep plenty of width in the limbs and tips until you have recurved it and then work on getting the string tracking straight thru the handle. Once the string tracks good you can narrow your limbs and taper the tips down. I like to taper the last 1/3 of the limb and leave the rest with paralell sides to the fade-outs. For me it is easier to get a good tiller by leaving the limbs wide and adjusting thickness and then narrow the sides to adjust the draw weight you are looking for (thickness for tiller/width for weight).