Maybe you can take a small clean bamboo skewer, usually used for barbecues, or any other rod made from some soft material, and measure the distance from flange to ground glass directly with it? I think it would not harm a delicate GG surface, just one little touch of a wooden stick? I remember myself doing it with no problems. Or if you have a rollfilm back like Super rollex, you can put it on your camera instead of your GG, open it, and measure your distance from film plane to lensboard flange with measure tape.
Good luck, Zhenya
