No reference because you can't tension it. Basically it's just a loop around the two pulleys and tension is determined by the belt length. For that reason it's as well to keep an eye on the belt at an annual service but otherwise it's fit and don't worry.
You don't actually need a lot of tension on a water pump, just enough to spin it and that's why the water pump usually lasts for much longer on the Europa TC than the Elan with the same design. On the Elan you're driving the alternator/dynamo as well and folks generally run with a tighter belt to accommodate the extra load.
Brian