Sorry John, but I'm confident that I've never responded to such a "rule of thumb" inquiry regarding walnut diameter growth decline as a function of tree height.
However, the link listed below is to a USDA Forest Service publication on the "DIAMETER GROWTH, SURVIVAL, AND VOLUME ESTIMATES FOR TREES IN INDIANA AND ILLINOIS." It includes estimates for black walnut.
http://www.nrs.fs.fed.us/pubs/rp/rp_nc257.pdf
Best of luck!