I misplaced my manual for this bike and want to adjust the chain. Can anyone tell me the correct tightness? Thanks.

 

The chain will get tighter as the swingarm flexes, so the main thing is to be sure that when the suspension bottoms out that the chain is slack enough to not put stress on your countershaft sprocket.

Have a couple of your fat friends sit on it and fully bottom the suspension, and then adjust it so it still has a bit of slack....but better a little to slack than a little to tight!