As the correct tension is a experience sort of thing just pop along to a few garages with the cover off and ask them what they think, but as a general rule a 1/4 twist with moderate thumb/finger pressure.
I've seen from actual slapping around to creaking when the engine comes to a stop on belt tightness!!
It's a hard thing to describe imo accurately, as each persons "moderate pressure" varies, so never hurts to ask others to look at the actual tension on the car so to speak.
There is another way and that is to check the deflection of the belt between the two cam pulleys, from memory it's around 8mm - 10mm.
tabetha