How is your eye sight? if it is good, and the diopter is set correctly you should be able to tell visually if the focus is right. If what you see is in focus and the images are not then may be something is wrong within the prism or the mirror or the film pressure plate. I do shoot F100 regularly and never had a problem with focus, but I remember I did have a problem with a Pentax 6x7 that gave sharper images when tilted down and a little out focus otherwise. I changed the prism seal which allowed it to sit tight and the problem was solved. Of course you have a fixed prism here but still the idea is that if the visual is different than the captured then it's either your sight, the prism, the mirror or the pressure plate.