I found this new 6x12 back slightly out of line with the ground glass markings - about 2mm left and right. The best thing to do is cut a mask out of some stiffish black plastic - I used a dvd case. With a bit of care, and a sharp knife it's possible to cut it so it pushes into place over the gg with the exact sized hole, so you see what the film will see. Composing with this mask on is brilliant, gets rid of every thing apart from a 6x12 portion.