There are a few important variables to consider. What is your film format, in other words, just how
big do you need to see the details? There's a significant difference between a 4x5 and 8x10 groundglass, for instance. Next, do you use a fresnel with the groundglass or not? (I don't - I hate
them - but if you do, there is only so much detail you can realistically resolve with a higher power
loupe). I have used GG loupes all the way from 4X to 10X, but generally prefer something mid-range
for versatility. One of my favorites is the Horseman 7X loupe - a nice compromise in size and
magnification. I presume it was actually made by Peak.