Need some help!!

I'm new here, .. great site and great resource.

I'm using the Weldon style holders for my ball or "lollipop" tools
I use for 5 axis cnc on aluminum.
The tools are 8" and 5.75" hangs out of the tool holder.

What I've found is if I over tighten the set screw too much it
causes a ton of run out.

I'm using either Accupro or Kennemetel tool holders, ..

"tight" and I have .001" of runout, .. take an extra "grunt" on
the set screw, and it'll jump to .006" - .008"

What's causing that, .. and just how tight does that screw really need to be?
Seems really odd to me.
I do grind small flats on the tools as a precaution, .. and perhaps I shouldn't???

Thanks,

Curtis