Well, here's a rundown of my experience in using graduated neutral density filters with the Mamiya 7.
First off, I use GND filters with a soft edge. Placement is not nearly as critical as when using a hard edged GND filter. After I have my scene composed, I determine how far down I want the effects of the ND filter to occur and then transfer that to the filter placement by looking at the front of the lens. For example, if the top third of my image includes the sky that I need to tone down, I slide the filter down one third of the way over the lens element when viewed by the front. If I were to err in one direction, I would tend to lean toward having the filter cover more of the lens, rather than less. There is actually quite a bit of leeway when using a graduated filter with the soft transition. I use this method quite frequently with my 43mm lens with good results. Hope this helps.
Matt