Sorry for being so slow Dr Love. You are correct of course! While the often quoted recommendation is 30-35% of travel (10.2 on our beloved DR 650). There isn't a real hard and fast rule. 100 mm or about 4" is often used on dirt bike rear suspension as a starting point. At the end of the day, rider preference has a lot to do with it. I personally like the bike with a good bit of sag at the rear, it just feels better to me. 4" feels much better than 3" and makes the bike a little lower (I am close to 6'2"so this isn't a BIG deal but I like reaching the ground on those side hills).
I would love to hear what springs/ vs sag/ vs free sag some of you folks are running.
Here is a photo of the DR shocks I have (ready for anodizing) from this past Monday: