http://www.hasselbladhistorical.eu/HT/HTCuC.aspx

there's the calculator thing...

I have a polaroid back, but that pretty much limits me to testing an exposure using 100 or 3200 speed film (unless I want to do some sort of equivalent exposure thing), and i've noticed that the fuji packfilms don't do long exposures gracefully. Also, the 100 looks better shot at 80, so i'm not sure if that's the film or my camera needing a CLA. Negatives look perfectly exposed, slides have yet to be developed.