Found some instructions. You will need the spanner wrench.
Adjustment range is approximately 4 full rotations.
1.) Loosen pre-load adjustment lock nut. (re. to fig. 1)
2.) Using a special tool such as our ZPN 750803 Softail
shock pre-load tool, turn adjusting plate clockwise to
stiffen pre-load & counter-clockwise to loosen pre-load.
(re. to fig. 2)
3.) Lock down pre-load adjusting nut using a medium
strength thread locker such as Loctite “Blue” or Permatex
“Blue” when pre-load has been set
Here's a link
http://zodiac-nl.hu/instructions/243067.pdf