When calculating aperture widths using the formula Focal Length/F Stop, should the focal length actually be the flange plate to focal plane distance? The reason I ask is that I'm doing a Nimslo panoramic conversion, using a Bronica 40mm lens, and the flange plate to focal plane distance is 76.5mm. I used 40mm in the aperture width calculation and have ended up with a roll of underexposed film. It would seem that I should use 76.5mm as the focal length (possibly?), but I've not been able to find any information about this. Anybody had any experience of this?