For total sag I typically use 28-29% of actual (not claimed) suspension travel. Actual is always a bit less than claimed, because the spec travel includes compressing the topout springs completely, which is not a real world situation.
On the first gen Stroms actual travel is ~140mm, so 40mm is a good sag number.
It's important to realize that just because you can get to a "good" sag number, that doesn't mean your suspension is ok. Bikes with very soft springs often are setup from the factory with a lot of static preload, and because of that have total sag numbers that look reasonable. The ratio of bike sag to total sag will tell the true story.
In general, more than a little bit of preload is bad, because it causes a step function in the forks compressibility whenever the fork is topped out. All that preload force has to be overcome in order for the fork to start moving again, which sends a large quick jolt to the chassis.The goal in choosing a spring rate is to have a spring stiff enough to prevent bottoming while using a minimum amount of preload, but soft enough to let the suspension use all it's travel.
'07 DL650, '01 DR-Z400S, '99 SV650 (race bike), '80 GS1000S, '85 RZ350, '08 Ducati 848