I've got an 8x10 camera arriving in the mail soon and need to buy a lens...

My rough calculations show the film diagonal of 8x10 film to be 322mm... what is the minimum extra coverage I'd need from the image circle? I have my eye on a lens that has 420mm coverage. Would the extra 100mm (10cm/4 inches)be sufficient for movements needed for general landscapes? I mostly shoot portraits, but am hoping to get a lens with the coverage required to do some movements for landscapes. It's a Caltar SII 300mm I'm looking at, by the way... and I know that's not very wide :-)

Thanks a million :-)