The main problem is the same one that occurs adjusting tappets on OHV engines. When you tighten the lock-nut, it pulls the central screw through the fixed nut until it is hard up against the upper side of the thread of the fixed nut. This always changes the adjustment from what seemed right when the lock-nut was loose or only slightly firm. The problem gets worse as the machine gets older, because there is more wear on the threads, resulting in more slack.
You need to understand which way the adjustment will shift when you tighten the lock-nut, and allow for it. You should also make your initial tests with the lock-nut moderately firm, and don't over-tighten it when you finish the adjustment - over-tightening accelerates the wear on the thread and makes it harder to adjust next time.
