find the longest section of the belt between the pulleys and press against it, it should not deflect more than 1/4 inch. I'd say apply moderate to light-moderate pressure on the belt.
I guess if you could translate the pressure into ft-lbs it would be around 5 to 10 ft-lbs.

I just loosen the alternator screw, use a bar and apply pressure against the alternator till the belt gets all the slack out of it and then use my freehand and feel the belt, while pressing on it, i tighten it till it only moves about 1/4 inch and then lock it down there.