If you lost tension after backing it out a half turn, then your preload adjuster was probably completely dry of fluid. Do a search on how to refill that, and then set your preload at the appropriate sag level.
For what it's worth, "proper" suspension sag is generally considered to be approximately 30% of suspension travel (i.e. when you sit on the bike, the suspension should be compressed about 30% of its travel). Given that the 1300 has a rear suspension travel of 4.5 inches, then you should have approximately 1.5" of travel used up when seated on the bike in your full riding gear. (Yes, I know that 1.5" is actually 33% of 4.5", but I'm going for easy math here.)
On my ST1300, compressing the rear suspension 1.5" (measured at the rear axle to a point on the bike directly vertical) makes it so that there's exactly 12.25" of clearance from the ground to the bottom tip of my rear fender. This is the reference that I use when I'm adjusting my rear preload for various load conditions - always get that rear fender 12.25" off the ground.
Once the rear preload is set correctly (and, as a result, the ride height is where it should be), you'll be surprised how easy the ST is to get up on the centerstand.
In addition to proper ride height, proper technique is important to make getting up on the centerstand possible. You should not be "lifting" the bike at all. You should be letting your body weight do the work.