The shelf was cut in about 3/8", I didn't exactly measure it, just starting filing away until it looked good. I learned the hard way from a previous bow not to go to the center. The bow broke in have right in the shelf.
The ipe was purchased at a local hardwoods store. They didn't have much in stock. I bought two 2 X 2's, 8' long so I could make several bows and tinker with the design a little. The backing was purchased on-line. I would like to make my own backings but the last time I purchased hickory at the hardwood store, the clerk went on a rant when I told him I wanted hickory and not pecan. He said there was no difference. Subsequently I had another bow fail when the backing snapped in the handle area.