I'd highly recommend getting a helper and setting the sag levels based on measurements, it's one of the easiest free gains you'll make in terms of ride quality and handling. Even without a helper it can be done using something tied around the fork upper, the rear is a bit tricker but can be done with a piece of weighted string that passes through the axle line as close to vertical as possible. Helper is far better though.
This video explains the more accurate method to measure your actual amount of sag, if you're not race fussy then you can just take one measurement and ignore friction.
Do note that they are aiming for 30% of 120mm hence aiming for 40mm of sag. Your Strom has 150mm front and 160mm rear I believe (check this in the manual) so you'd be aiming for a total sag from completely unloaded of 50mm front and 53mm rear. You can run a different percentage based on your riding but the key is getting the front and back to match so the bike geometry is correct.
Not tried setting up with pillion but I do have settings for loaded and unloaded which are easy enough to switch between for touring or a solo day out. I have them written down and then try to stay out of the pie shop so they don't change
For rebound I just ride along in a standing position and bounce my weight directly downwards through the foot pegs, I try and match the rebound rate from front to rear so it feels as though the bike bounces back up evenly. Since the front is not adjustable but the back is, I figure this is as good as I will get it without messing with fork internals.
Happy fettling, you won't regret the time and effort believe me.