How to properly focus using a Macro reverse ring?

If you achieve proper focus you will be able to see a clear image in both your optical viewfinder and your Live View display.

The fact that you don’t see a clear image is just because you do not have proper focus. It is just blind luck that when you take the photo, you see a slightly better image, and that is probably just due to Depth of Field.

Most people reverse shorter focal length lenses like a 50mm for Macro work. My guess is you are too far away from the subject when using your 55-200 lens. Try shooting it at 55mm and move the lens closer so it is almost touching, and then very carefully increase the shooting distance until the image is clear in both your optical viewfinder and live View display.

If you are still not able to achieve sharp focus, it is because the focus point is actually inside the lens, and it will be impossible to use that lens reversed.