I have a Linhof Techno digital view camera and made the discovery that 6x or 10x lupes is way too low resolution to maximize ground glass capability and focus precision. The 6x lupe tradition is obviously for 4x5" not for 36x48mm digital backs. Using a 1:1 macro lens on a Canon 5D mark II to look at the ground glass with live view I see more detail and considerably better precision in focusing can be achieved than with my 10x lupe.
For the 36x48mm area the ground glass shows detail corresponding to about a 18 megapixel sensor, and I've calculated that about 30x magnification would be suitable for critical focusing.
Here's a photo with 100% crops http://torger.dyndns.org/temp/dwcf-fs.png showing the groundglass as the 5D sees it at roughly 4000 ppi and the resulting 33 megapixel picture. With the 10x lupe I can barely see that there are numbers on the slanted ruler, but I can definitely not read them.
Has anyone experimented with higher magnification on the ground glass and have tips of what to use? I'm thinking of a high quality 25x pen microscope, not sure if it will work though.