While using Kiev 45 prism on Hasselblad, I found that is way easier for me to focus after I added -1 diopter to it. I'm wearing glasses all the time, -2.5 diopters on right eye. Took prism to local optical shop (the one where they rally know how to cut real glass) and they did it. Not expensive at all. After that it was way easier to use prism with all kinds of Hasselblad focusing screens. YMMV!