IMO, That blade should stop anywhere, not in just one position or a certain one. If you let go of it and it stops at a certain spot every time, it is out of balance. one of the reasons I went to a electric fan. Severly out of balance mechanical engine fans can rattle stuff to the point of failure or shorten there lifespan dramatically. I don't thing manufacture's take much pride in seeing engine fans balanced properly. stamp them out, stuff them in a box. Maybe check one in fifty, or one in a hundred.
With that being posted, I don't think there is to much you can do about it in the garage. To be properly checked and balanced, it need to go to a airplane prop shop, where it is balanced on shaft mounted cone arbors, free floating magnets, and ceramic bearings,,and that is big bucks.
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