You should be able to lift the chain with your thumb and forefinger at least an 1/8" off the bar. It should snap back when you let go. If you can't lift it at all, its too tight. Make sure when you tighten it you don't tighten your bar nuts all the way. Snug the bar nuts, pick up the tip of the bar, and tighten your tensioner while you are holding the bar tip up. If you pass the snap test, cinch down the bar nuts and away you go. If you cinch the bar nuts down first, your tensioner will not work.
Everything Joe said. Trying to move the chain around with your fingers is asking to get sewn up. I can validate that claim. if the chain is too loose, you'll cut poorly, and if too tight you'll chew up saw bars and make your saw work too hard. I always have told guys too lift up those nose of the bar with the nuts loose, and tighten the chain just until the daylight between the chain and the bottom of the bar goes away. Keep the tip lifted with one hand and tighten the nuts with the other hand. Contrary to popular belief, those bars nuts don't have to be tightened by a gorilla. Snug em up good, and no more.