I second the suggestion to try a few, if possible.
I'll leave this thread with just a couple things - greater magnification is not always better, the coarseness of the GG is important and relates to magnification. I have a couple of the late Satin Snow screens and it was just too fine for my eyes (but so very well made). Finally if you use a GG with a hood as some field cameras have, be sure it reaches into the area properly. The later point is why I had to swing away the hood on mine, a real PITA. I lucked out in finding a used Wista loupe, the long, collapsible one; no longer made, I think.
Good luck!
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