Belt tension

So I've almost got it put back together, But I'm going to have to run a electric fan I think.
I've been looking around the web for the proper belt tension, and thought I found it the other day but can't seem to find it again. I thought it was something like 1/2 inch of slack in the belt when the tensioner is tight. Does this sound right. I got 2 belts along with this stuff when I got it, one fits ok like it was the one he used and one looks to be new. But it's a little smaller then the older one. I can move the tensioner to the next holes with the new belt and get the belt to fit but it seems tight. And with the old belt the tensioner is on it's last holes and tension is all the way out, but the belt seems tight enough, but no more adjustment. Any info on the proper slack. Thanks