Beadman, I had a similiar red mulberry stave a few years ago. After posting some pictures of the natural arch in the log I was discouraged from from attempting to preoceed any further. A couple inches of natural reflex can help a bow store extra energy, but there is a limit to how much reflex is workable or safe. A 6" in dip in a 6' log might be too much to work with. Pictures help.
Eric