One word for anyone having trouble adjusting their chain, shaft. Go buy an Explorer or Tenere or GS. I used to be a shaft only kind of guy, hated chains. BUT my wife has only ever ridden chain driven bikes, SO I was stuck with chain maintenance anyway. I realized a few years ago that it really doesn't matter what drive a bike uses, I just buy the bike that I want to ride. And modern chains are pretty damn easy to maintain.
I do as a few have suggested. Adjust on the side stand, put it back on the center stand and re-measure so I know where to adjust to next time. Also check the frame alignment notches or detents on a new bike to see if they're accurate.
Best way to check tension is as one poster mentioned, line up the axle, swingarm pivot, and front sprocket. Use a tie down over the seat to the swingarm. Once they are in line you should have just a slight bit of slack, about 1/2" is plenty as the chain can never get any tighter at that point. Release the tie strap and then put your bike on the side stand, center stand, whichever you prefer and use that as your chain tension.
On our sons dirt bikes we just used four fingers between swingarm and chain, now those were run loose.
How to do it with pretty pictures...
Great tip at the end for making a small wooden chain tension tester.