I currently own a Toyo 3.6x fixed lens focusing loupe. I wonder if the design of this loupe takes in to consideration the thickness of the ground-glass; and if not, could I gain a slight bit more accuracy by using a loupe that could be pre-adjusted on the grain of the GG's interior side?
With an adjustable-focus loupe, the way to set the focus is to remove the lens from the camera, point the camera at a bright window or similar, then adjust the focus of the loupe so that the grain on the GG is in best focus. You could check that your Toyo loupe is in focus on the GG in the same way. You have no real recourse if it isn't though - maybe some sort of circular shim if moving away from the GG improves the focus.