A large amount of this discussion is unintelligible to me, but I'm glad people are sorting all this.

With all that knowledge, any guesses as to a reasonable sunny exposure time for Illford multigrade IV in a 8x10 camera with 6 inch focal length and .5mm pinhole (f/300ish)?

What I really want is just an ISO number. Since I know my aperture (or can find it), if I knew the speed of my paper I could just meter a scene with another camera and add stops for film slowness and aperture smallness, with maybe a fudge for reciprocity. Possibly it's not that simple, but if it's so complicated I can see just going for $3 a pop for sheet film, then at least I can get an ISO rating and reciprocity data that I can understand.