Each line of the cross hairs on the screen is actually two parallel lines, like this: ll The trick is to focus your eye on the screen so that the two lines of the cross hairs are sharp, and then adjust the lens while keeping the cross hairs sharp. Your image will be in focus when both the image and the cross hairs are sharp. Of course, all this assumes that the screen and mirror are properly aligned, but I suggest that you start here and see how things work. I hope this helps.
These old Hasselblads can help you make wonderful images if you have the patience to learn their character. Enjoy!
- John