There are no absolute measures, as each person likes their batter a little thicker or thinner etc etc, its all about how you like it. This batter will stay crisp even if the fish is allowed to go cold.
I was surprised by this, tried it twice, first with Fosters then with Guinness, didnt like the look with guinness was a strange colour and not as crisp as with larger. Maybe you should leave the stout option out? - 12 May 2014