How much preload you want depends on the springs in the forks, and the spring and settings at the rear.
I have a Ohlins on my PD with a custom made soft spring, probably around 400 lb, which gives me around 65 mm of well damped rider sag.
The forks have the softest factory spring for 36 mm forks with no additional spacers, and with 5wt synth suspension fluid and Sahara 3 s at 31/33 I am able to tune the Ohlins to them almost perfectly.
Floats over bumps with the legendary Ohlins Magic carpet, and well damped enough to hold a perfect line when cruising on our bumpy main roads at the state limit.
A SJ billet top triple, and tubular lower brace help here too as after 460,000 km the forks are understandably a little loose without them.
To advise on you spacer without knowing what you have elsewhere in the suspension is pointless, as in the final analysis what you are balancing is front to rear, rear around 10% stiffer than the front, and the clue that you are getting close is when a small change at one end makes both ends feel better.
FWIW I dont notice any difference in handling between a full or almost empty G/S PD tank , but the HPN inserts in my GS came with two sets of preload spacers, 10 mm longer for the PD.