Boxerworks.com

Many don't realize that tightening the center nut adds pressure (torque) to the bearings. The threads have a bit of play in them and that top nut removes it. That is just one more small factor that makes the job a bit fussy.

Don't even begin to think that if it falls from side to side easily that you have it correctly set. That would be a false positive.

If it doesn't fall from side to side, then you do have a problem. If you have play in the bearings, then as you hit the front brake, it will click and then you know. If it is too tight, then you won't be able to go in a straight line at low speeds. It will "hunt" all over the place.

You are looking for the spot of no preload and no play. We used a few "cheating tricks" to make the job go much faster.

The top bearing inner race should be able to slide on the steering head in order to easily assess the point of play/no play.

Just my thoughts, through observation.
I could be wrong.

I had a hell of time with this job, went through a couple of sets of bearings,
until
I bought a new lower nut
As Duane said the nuts have some play (lash) on the thread,
when you tighten that top nut it crushes down on the lower nut, and then its easy to overtighten those bearings.
I found its almost impossible to set right with a worn lower nut.
New nut, careful adjustment and I haven't had to touch it again for ... about ten years now.

So that's my tip.

cheers Mal R100/7