John, I agree with you. The fine side of my Tormek stone grader is dished also and having this shape it cannot correctly grade the grindstone. Oval dished stone grader grades the edges of the stone, leaving the middle part untouched. If I need to grade the SG stone really fine, not compromising its trueness, I use an inexpensive diamond plate (1000 grit) mounted in the square edge jig.
In my opinion the grader should be replaced as soon as it is visible that it is irregularly dished.
I am going to make simple bracket, which mounted in the square edge jig, would guarantee that the grader approaches the grind stone always in the same way.
Recently I have bought dressing sticks designed for refreshing diamond and CBN wheels. The sticks are made of aluminium oxide and should be used for cleaning and opening of the superabrasive wheels. They are inexpensive.
The large (30x30x300 mm), dark dressing stick is for bench grinder. It is made of silicon carbide.
Jan