set the rear shock pre-load to give 75mm of sag while you are seated..(( 35%/40% of suspension travel , the wheel
needs to have travel in both directions when you are seated on the bike))
or as close as you can get with your weight & the stock yamaha spring...
The chattering over small consecutive bumps is something you have to live with , as the XT shock is rubbish..
But the correct sag will make the damping a little better as the shock is not fighting against a overly high pre-load setting..

The max pre-load on a shock spring should be no more
than 20mm between free length & max pre-load compression..

I set my bikes at 1/3rd "30%/35%" sag of wheel travel
with a linear progression curve..(endruro set up rarther than MX)
XTR @ 75mm rider Sag..
WR250F @ 100mm rider sag..

__________________
Harley Davidson..
The easyest way to turn gasoline into noise with out the side efect of horse power...