I've not yet had to adjust the chain on my XT but my last bike was a Yamaha and whilst at my local dealer with my previous bike, I spoke with one of the technicians about chain adjustment. He told me to leave the wheel on the ground, push down on the chain at (roughly) the centre point between the sprockets (below the swing arm). Then lift it up. The measurement you are looking for is between the top of the chain when you push down and the bottom of chain when you push it up.
Like I say, I've not had to do it on the XT so it may not work, but that's how he said they adjust chains in their workshop