Just to reinforce what the others say, a little too slack is better than too tight.
Easiest "rule of thumb" guide is as Mike says above, with the bike on the sidestand you should just about be able to lift the chain midway along the lower run to touch the chain rubbing strip on the underside of the swingarm. As Mark says, with a ruler held beside it that should mean about 35mm of total up/down movement with light finger pressure.
While marks on the adjusters in the slots next to the wheel spindle can be out, it's the easiest and most convenient way to check the alignmen