A member of the large format forum sent me some pictures of the aperture lever at both extremes of travel and also one of the front with the front lens elements removed so I could see the aperture blades relative to the rear lens elements when set to f8 (i.e. not quite covering any of the lens.
My conclusion now is that the scale I have fitted to the shutter is not for the 80mm lens elements it had fitted to it as my actual f8 position is half way between f5.6 and f8 on the old scale rather than between f8 and f11 as previously calculated.
The elements this shutter had are an 80mm Componon which is an enlarging lens which wouldn't normally be fitted to a shutter so I now think that it was wrong to assume that this scale matched the 80mm lens.
Anyway, it's now time to use it!