If you really want to be sure you have just the right amount of chain slack then lift the bike off the ground and disconnect the shock from the swing-arm by removing the dog bone links - and then you can easily lift the back wheel all the way through its range. This way you can ensure that at the tightest point of the arc the chain is not pinging tight.

I raised my bike with shorter dog bones, which exacerbates the amount of chain slack when my bike is unweighted - but swinging the rear wheel thru its arc shows that it needs that extra slack!