Calculating aperture width

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?