Put the bike on a stand with the rear tire off the ground. Measure from the axle to some point on the rear fender. Make sure your measuring points are repeatable. Take the bike off the stand and have the rider climb on fully geared up. Have the rider bounce on the pegs once or twice and sit as neutrally on the bike as they can while just holding themselves the bike steady with a toe on the stand. Measure the saq. You may want to repeat the bounce and measure thing a couple of times to make sure you have a consistent number. 

That's your race or rider sag. I always aim for 95-100mm but others like different numbers. If it's not at whatever number you're aiming for, you need to put more or less preload on the spring. Sometimes you can do that with the shock in place by loosening the allan bolt in the collar and turning the spring. I usually just take the shock off and use the KTM wrench to adjust it. It's only two bolts to remove the shock and it slides right out the bottom. Shock bolts torque to 44ft lbs, BTW.

Once you have the rider sag set, measure the amount that the bike sags just under it's own weight. That number should be 25mm +/-10mm. If it's not, you'll need to buy a different spring.