So it doesn't matter which suspension you have, you do need about 1/3 spring preload in ride height.
Now that statement does not mean they are all equal, they are not that at all. It's just that ALL suspensions need to be setup the same, 1/3 - 2/3. Performance of each is then based on valving and spring rates etc.
It also depends on the total amount of suspension travel available, load, etc. If memory serves me, Bitubo recommends a starting point of about 25mm, which is right at an inch, so if the total travel of the shock is 3" (on their 330mm shock), that would be about what you are saying, and I think that is about what a "normal" height HD bike has, the shock being about 13" from eye to eye. The spring preload adjusts the sag on the Bitubos, just like any other shock...the difference is that they also have a 5 click (or is it 6?) adjustment on rebound damping, and a non-linear needle valve on the external canister that has about 13 clicks for compression damping, so that the shock won't bottom out. So, at a setting of say 7 on the compression damping for normal riding, you might have to adjust the preload on the spring a bit for a loaded condition, and raise the number on the compression damping to maybe 10 or so...the effect from 7 to 10 is greater for each click than it is between 1 and 6, if you get what I mean, which I'm sure you do.
However, IMO, the sag on the front forks of a touring model of a HD bike is waaaaay too much because of weak springs and thus you don't have the suspension travel needed to keep from bottoming out...again this is my opinion...you can practically bottom the front end of a touring bike by simply getting on the front brakes hard, which is unacceptable, IMO, because then you have little, if any, travel left to soak up any road irregularity encountered....that, along with piss poor damping control, leaves much to be desired, especially if you ride somewhat aggressively in the twisties. The front tire can only stay on the ground with proper springs and damping control...things like running over a rubboard type road surface in the middle of a turn can make the pucker factor increase exponentially when your front tire is skipping across the surface sideways. Springs are always a compromise. It's really hard to describe the difference the AK-20 cartridges make, other than to say your tire is in constant, controllable contact with the road surface at all times. The bike just tracks better, and brakes more effectively because the front end does not dive so dramatically. I'm not saying it becomes a sport bike, because it can't.