Grab the chain at the middle of the rear sprocket and pull it back, if you can expose the sprocket teeth enough to see light through them, the chain is worn too much and needs to be replaced
Then if you want to prolong your chain life and set the slack 'correctly', put the bike on some form of stand, get the rear wheel off the ground. Remove the lower shock bolt and then zip tie the shock up and out of the way. Move the swingarm through its stroke, chances are you have your chain too tight and you'll feel it noticeably tighten up when the swingarm is horizontal. You need to set the chain to be as tight as it can be, but not develop a tight spot in the swingarm stroke. Otherwise it'll put huge stress on the chain, sprockets, seals and gearbox. You'll ruin a chain and sprockets in no time if that's the case
Once you've found the tightest spot it can be, then you can do your measurements for how much slack there is when its just sitting under its own weight. Record that and set it to that from now on
P.S. if you are replacing a chain its good practice to replace the sprockets as well. Doesn't matter how tough the chain is, if the sprockets are bent or worn then the chain will inevitably have to reshape itself to match the sprockets (and vice versa). Ergo it wears out a lot quicker
