Reuben,
You seem to be on a quest for sharpest focussing ability for your camera.
The "tape-the-focussing-screen-of-your-choice-over-the-film-rails" method has already been described. But you might take this opportunity and buy a few recommended types and brands, to see if you have a preference.
Alternatively, you might do a film test. For a given lens, then for each aperture, take 3 pictures, one focussed on the subject (brick walls are good, many levels of detail), one focussed slightly in front and one focussed slightly behind.
Compare results. They may give you information about your system that will aid you in making a choice.
I have a diopter attachment for my Kiev-4, but no lens for it. Perhaps Edmund Scientific has something that might fit. I also have 2 for my Zenits and 1 for my Pentax (only fits my Konica). The rubber eye cups on my Kiev 60 prisms do not come off easily, so I use nothing on my K60.
All my diopter attachment are about 2x, and I use them when I need to get better focus. The focussing circle appears much larger, and FOV is therefore much smaller, but getting and verifying correct focus is much faster and easier.
I do not necessarily see sharper results on film, but I see very few out-of-focus, poorly focussed, or incorrectly-focussed results. This way, I avoid the focussing-screen-type question.
