Computing coverage is reasonably straightforward but one of those things which is simpler to just try and see. You can use the lens equations to get the image distance - 1/subject distance (8 ft in your example) + 1/image distance = 1/f where f is the focal length). Now if you know the coverage at infinity i.e., when image distance is 1 focal length, you can use the laws of similar triangles to derive the coverage at this new focus distance. In other words, coverage at infinity/image distance at infinity (i.e., one focal length) should be equal to coverage at 8 ft/image distance at 8 ft. You've calculated image distance at 8 ft from the lens equation. Plug and chug....

I think at the 210 end, you can get away with 8x10 coverage as long as you focus close and don't have much movement demands - corner might be problematic. Given that 300+mm is considered normal on 8x10, 370mm may still prove short for portraiture. Also, resolution at 370 is not as good as at 210 but this might actually be a benefit for portraiture.

Lastly, try using the front element of your lens i.e., removing the rear element. By my rough calculations thats roughly a 485mm f/16 lens you end up with. I have used this lens in this fashion. Resolution can be considered appalling but mine gives me very nice soft focus effects, a little difficult to control but sometimes pleasing....

Hope this helps. DJ