About loupes. I often have used a 50mm lens, looking through the front. Just move it back and forth until you see the ground glass surface clearly, I think it would be about 5X at that point. My magnifiers are 'as found' and I've also have a 80mm viewing lens from a wrecked TLR. For even closer I use a 34mm lens that was a projection lens from a microfilm reader. The plastic calbration screen does not have a very fine surface so the hi mag 34mm lens is not of much use. The 80 seems about right for this screen.
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