First of all, as you probably have figured out, you need to put a lot of pressure on the stone grader. Whatever way you can pull that off is just fine. If you do it perpendicular to the wheel in the center of the stone grader eventually the wheel will wear a groove in the stone grader and conform to the radius of the wheel. That's OK. After a while you can then move to either end.
If you do it parallel to the wheel, that works too, but is hard to hold. Really it's whatever works best for you.
Personally, I went to using the edges of the stone grader and pressing very hard just to get the job done a quickly a possible. I'm guessing you will just end up using the grader in a variety of positions, just to get the most out of it before it needs replacing. It's another consumable.
I wouldn't over think it. Just experiment with it and do whatever works for you and you will be fine. No matter what I say, that's what you will end up doing anyway.
If you do it parallel to the wheel, that works too, but is hard to hold. Really it's whatever works best for you.
Personally, I went to using the edges of the stone grader and pressing very hard just to get the job done a quickly a possible. I'm guessing you will just end up using the grader in a variety of positions, just to get the most out of it before it needs replacing. It's another consumable.
I wouldn't over think it. Just experiment with it and do whatever works for you and you will be fine. No matter what I say, that's what you will end up doing anyway.