Daryl,
I think you are right, but not too sure at what level would relate to a mid-range for the 600. I might have him (gunsmith) load higher, and with some 4759 or 3031 as well. He has only built the 450s and 577s and not a 600, so his start from loads for regulating it is going to be new for him. I also want to keep recoil low enough for my wife to shoot upon occasion as sshe does like my 577.