the instructions in the owners manual are the best.
Midway between front & rear sprockets, the bike on side stand and then adjust. I set mine to 40~45mm

Remember if you put any weight on the bike (like sitting on it) you can no longer use the slack stated in the handbook.

You need good slack if you go off road as the XT has a long suspension travel.

My tip: put a small wooden block under the stand to make the bike more upright