0.60 logD over paper base is the ISO paper speed point. My 500H, when adjusted, seems to produce this midtone at all grades at the same exposure, for a given negative, which I believe is the meaning of a speed-matched system. This agrees with what I read in Ralph's book, which shares useful suggestions how to take this fact further, and to arrive at an equal highlight while changing grades, which may be more useful than trying to keep a rather dark midtone constant.