Tim, what zoom do you typically have the 580s set too?
35mm. Go on, ask me why. Why? Ok, i'll tell you. Just because. No good reason.
Really though, it varies a little depending on the room. For really tall rooms i'd use 105mm, for really small i'd use 24mm, for most i'd use around 35mm. I don't know if it makes a significant difference, but I just work out what area of the ceiling I want the light to come from, sight it through a lens (or guess), and set the zoom.
Try it out for yourself and see if it makes any difference.