Thanks to Regina Chain:
The chain will be tightest when the three pivots line up in a straight line
The countershaft, the swingarm shaft and the rear axle. If you can compress your suspension to that point and verify there is still a bit of slack you'll know you have it right. Then release the suspension and see what it needs to be when the suspension is unloaded and make a mental note or take measurements
1 First of all, the chain must be placed at its maximum tension condition. This condition will occur whenever the engine and wheel sprockets are at the maximum distance from each other.
To obtain this, it is generally necessary to load the back wheel (for example placing a person on the motorcycle) until the centers of the engine sprocket, the swingarm pivot and the rear wheel axle have been positioned on the same axis. The swinging arm will be parallel to the ground.
More pics and info, thanks to TrophyHunter: