After my original chain jumped the rear sprocket, (expensive!) I'm paranoid about a chain too slack.

I adjust mine by slowly tightening it up, test riding it until I get the whine of a too tight chain and then slacken it off 2 sides of the adjustment nut. As already said the tightest point is when the centres of the front sprocket, swinging arm pivot bolt and rear axle are all in line. So you could disconnect the rear shock and move the wheel through this point and check the chain there.