The service manul says to place the bike on the sidestand when adjusting the drive belt tension.  What is wrong with having the bike secured in the upright position when making the adjustment?  It would seem that this would provide the most accurate result.  (Also easier to read the scale on the frame.)

the difference may be the rear suspension preload, on the side stand virtually none, straight up perhaps some, rear wheel in the air, a negative amount. is it enough to effectively change the distance/tension on the belt? probably not, with out checking in each condition, and reporting back i wouldn't know.

Greg D showed me how to adjust my belt and he did it while on the kickstand.  I have done it several times myself now and have had no problem. By the way, chuck the tool that comes in the tool kit for adjusting the belt. All you need to do is be able to bend the belt at a 45 degree angle. You dont want it tight.  Mine has enough slack init the belt will go up and down with at least a inch play.