Measure the distance from the rear axel bolt to a good point up around the back of the seat or tail rack when the rear shock is fully extended. Then add all your luggage and gear and sit on the bike with your full weight. Have someone measure the same two points. The difference between these two measurements is the "sag". The sag should be around 3.5 inches. IN other words, the rear shock should not compress more than about 3.5 inches from fully extended once you add all the weight you will be carrying including yourself.
If it compresses more than this, you should add some pre-load to the rear shock.
Some folks will like it more like 3 inches for knarly off road and jumps and such. 3.75 inch for hiway should give a softer ride, as long as the bike is still stable. Too much sag can cause teh bike to become unstable.
Ok, I am no expert. So you experienced folks should chime in here and correct anything I have stated that is incorrect. I am just repeating what I was told.