Loupe for GG?

I have a large Promaster 3.5x loupe; it was actually designed for 35mm slides, and can cover them completely; I use that for my 4x5 loupe right now when focusing on GG. It is nice in the sense that it has no distortion when looking straight-on and can look at a lot of GG. However, I can't view corners with it, and I'd like more magnification.

I have very finely ground GG, hand ground by Steven A. Hopf (Ultra Fine). He sells on eBay.

Given that, what would be the largest magnification for loupes I could reasonably use?

Loupe for viewing transparencies?

For viewing transparencies and negatives, I figure that my sharpest lens can deliver 100 lines/mm to Pan 25 B&W film. Assuming that I want to look at the film at 5 lines/mm (the lowest resolution I'd consider printing at, according to studies of what human eyes can discern), that means I'd need a 20x loupe. Any suggestions for a good one?

Basically, I want to use this loupe to (a) enjoy my transparencies / negatives; (b) Figure out how sharp they are, thus how much resolution to scan them in at. (would it then make sense to get an intermediate loupe between 5x and 20x, say 10x)?

Any suggestions on loupes for this purpose?

Worth considering this loupe for examining transparencies? 40x multifunction macro scope from eBay