Just to clarify, rear sag adjustment should be made with the weight on the bike, not lifted with weight off?You can adjust preload on the ground, and you'll need to for the rear spring anyway.
The only reason the bike needed to be up was for old school sag measurements.
The Slacker shouldn't need that.
And, do you have a piece of 2' or 3' pipe you can get on your spanner wrench?