A firm I used to work for used one of those, but I believe we got the correlation from a GT engineering book that belonged to an engineer that my boss knew. Good tool for when the boring trucks are two weeks out.
Keep in mind making recommendations or designing from correlated CBR values can be risky, as the in situ field moisture content plays such a large role in the DCP values obtained. As you know lab tests are tested at optimum moisture content (or close too) and give much more reliable data. I understand it is hard to get CBR tests sometimes though!