"Isn't the rule of thumb that a process lens has a coverage area two times the diagonal of the film used?"
Nope. Most process lenses have relatively small coverage. Published coverages are usually at 1:1, and are twice the coverage at infinity. For Apo-Nikkors' coverage, see http://www.galerie-photo.com/apo-pro...ikkors-en.html
In addition, the angle covered is smaller for long lenses than for short ones.
That said, there are wide-angle process lenses, e.g., G-Clarons, G-Claron WA, Apo-Gerogons, ... These typically cover at most 70 degrees.
The Cooke Ser. IX should cover 8x10 at infinity, but no more. It is certainly worth trying as a portrait lens on 8x10. Randy, what will you use for a shutter with it?