With the Derailleur Chaingap gap adjustment gauge, you can align your rear derailleur correctly.
On a full-suspension mountain bike, first, bring the chassis in SAG. Place the template on the upper pulley. Then turn the B-adjust screw on the rear derailleur until the longest teeth of the largest cassette sprocket are aligned with the sprocket outline of the tool.
The Derailleur Chaingap is handled slightly differently: Put the chain on the second-largest sprocket and place the template on the largest sprocket with the arrow pointing towards the upper derailleur pulley. Then turn the adjusting bolt B on the rear derailleur until the arrow on the template points exactly to the center of the axle of the upper derailleur pulley.