I have only reloaded for two Roys, a 7MMWM and 257WM, and never loaded Barnes bullets. So with that said, my advise is to load to the recommended COAL and see what accuracy that gives you and go from there, which is probably out further. You will never get anywhere near the lands in a Mark VI with standard length chamber, so finding the sweet spot for one hasn't really been dependent on cartridge length too much.