James Chow's suggestion of looking at an angle to the GG (when looking near the edges of the frame) with wide angle lenses is very important. As James suggests, you can use a loupe without a skirt so it can be held at an angle, or you can flip your skirted loupe over, and use it backwards. I use the Peak Anastigmatic 4x loupe for focusing most of the time. This is a rather large loupe with a fairly flat and wide field of view. To a limited extent while viewing near the edges, I can hold the loupe flat against the GG, and view though the loupe with my eye at an angle to the loupe. This has a similar effect as holding the loupe at an angle, but the whole GG area stays in focus. I guess what I'm saying is that you have to experiment and find what works best for you.