The Zero Image web page says to hold it 40mm in front of your eye for their 6x9 cameras. That should be the focal length of the camera. If you cut your own 6x9 cm frame and hold it at the focal length of the camera in front of your eye, it will frame properly. Think of it as placing your "eye as pinhole" at the same distance from the pinhole to film, and looking through a frame the size of the negative.
I use an Olympus OM body and 21mm uiko lens, which is just about the same field of view. Yes, it is the f2 . . . I didn't want one of those cheap viewfinders (the f3.5). Bill Barber