I just bought a Mamiya RB Pro-SD with a 6x8 power back. Looking through WLF, I see the normal guide lines for vertical orientation. Here's the question. As I understand it, these guidelines are for 6x7 film back. I have a 6x8 back. How do I know where my real guideline is for this back?? I am aware, I get full 6x8 on vertical position only.