I support the bike by the frame, uncouple the linkage/shock, lift the rear wheel till the chain slack is at its least amount (you'll feel it go over centre when it loosens again). Support the wheel with chain at its tightest point, then adjust the chain in this position so slack is at a minimum, turn the wheel to a few different points and check slack in case of any sprocket high spots. Re assemble then put the bike on it's centre stand or paddock stand in my case as somebody chopped it off, pick a reference point on top or bottom of swinging arm immediately behind chain wear pad. Pull the chain away from the swinging arm, this is my correct chain slack. I then cut a wooden block to this thickness to use as a quick check gauge until chain/sprocket renewal. Make the block small enough to keep in your pocket or stash under the seat.
This has worked fine for all my bikes except the Moto Guzzi.