I just installed a Craftsman Garage door opener at our house, chain drive. The manual said to have enough slack so that the bottom part of the chain was not below the bottom part of the steel support channel. I initially set it up correctly and over a period of a month or so I had to re-tighten everything because too much sag developed. I just adjusted the tension at the door end where the chain attaches to the bracket linking to the door.
So overall I would say you want there to be enough tension to keep it just about even with the bottom of the center steel beam. This is of course at the lowest point of the chain.