ahicks said:10% of the suspension travel works pretty good as a general rule of thumb. A lot may depend on how aggressively you're running too. If you run one those all day rides you may want more sag than for an afternoon romp in the bumps.
Agreed, likely one of, if not THE most important setting there is.
Too stiff (too little sag), and you're only using the top of the suspension travel, too little, and you're only using the bottom.....