Now I understand the instructions; they want you to measure the clearance when you collapse the lifter. You could do that, or you can do what you said; loosen up all the nuts and set to zero clearance, plus 3/4 turn. I should have said "rock" the rocker when I said "move" the rocker to check clearance. There will be some side-to-side looseness due to the clearances in the valve alignment devices, whether it's a set of rails on the rocker arm, a slot in the head, or guide plates. The bolt-down pedestals work with sled-type rockers, so there should be no side-to-side motion.

If you have a bolt-down pedestal rocker, it's obvious. If you have a nut on a stud, you can take off the nut and see if the stud body goes straight down its entire length or has a shoulder. The shoulder is the positive-stop, and that version cannot be adjusted. The other type is adjustable.

If there is too much rocker to stem tip clearance, the tip of the arm will tap against the tip of the valve, which you will hear with the engine running. If you do not turn the nut down enough, it can loosen up with the engine running, make really loud tapping noises, and possibly come off the stud and cause all sorts of damage.

If you tighten it too much, it will not allow the valve to seat properly, which will cause the valve and seat to burn, the engine will back fire and run poorly.