Try various approaches and see what works! Seriously. I had been using digital cameras since getting back into photography 10 years ago. Viewfinders and diopters were either not an issue or built in. Being over 40, my eyes have been slowly losing close focus. So when I started using an older film camera last Spring, I had problems.
Problems will be pretty obvious. Not being able to read viewfinder displays. Or not being able to tell if something is in focus. It isn't as if your focus will be shifted 3% front or back; this isn't like autofocus front/back focus. This is simply less than focused.
My answer can be seen here:
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Realize that off-the-shelf readers work fine for me. I took the camera into a store and just tried a variety of strengths until I found one that gave the best focus. I bought the pair and used it to make the eyepiece.
Just try different glasses, lenses, etc. I've wondered if I could take an eyepiece like I used into an optometrist, try various blanks out of their stock, and have them cut to fit.
FYI, my usual reading glass strength is +2.25 or so. For the eyepiece adapter, +1.25 is what actually worked best. So simply using your standard prescription might not be the best lens. Maybe there is a logic to how much diopter offset will make one work, and then you can get a prescription lens with the astigmatism correction, also?
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