fork spring preload?

Easiest way is to clamp the fork in a vice horizontally with soft jaws by the lug and slide the upper tube down all the way until it bottoms out. Measure the distance from the top of the tube to an easy to mark place on the cartridge and record the number(for example 90mm). Now remove the rebound base bolt from the cartridge and take the same measurement again with the cartridge just resting on the spring(this time say 96mm). Since the fork is horizontal there will be no weight from the cartridge compressing the spring and that will be 0 preload. Subtract the installed number from the un installed number and you would have 6mm of preload.

How can the preload be adjusted? By a different length spring or?

Racetech gave me little washers/spacers to put under the spring