Just thought I'd pass on some really wonderful advice I received over the weekend. While shooting with a 4x5 and 90mm, f8, I was having problems seeing part of the image on the ground glass. A bit too dark. A person watching heard me mutter from underneath the focusing cloth and asked me the problem. I told him.
His advice: "If you had one of those little maglite flashlights under there that would help. I carry one on my keychain"