Correct me if I'm wrong but I'm of the impression that flare values are difficult to predict - what the charts always give you is average magnitude. Rocket bodies show a lot of variance in brightness because they're literally tumbling. The 3rd stage, which is the one all the observation programs tell you about, is 3 meters wide and 15 meters long, as well as bright white. francopoli care to weigh in?
Iridium flares are fantastic if you catch one at an outreach event. We had a bright one, -6.8 that lingered above us for 2-3 seconds and then followed the satellite for several more seconds after that. We had a crowd of about 50 or so around us and they all "OOOOOHHHHH AHHHHHHH" with a few "WHAT THE FUCK WAS THAT!" for good measure. There is an android app called ISS Detector that is well worth the money. It pulls your GPS and calculated the next 10 days if ISS and Iridium flare sightings. As far as the brightness, the closer you are to the central part of the flare path, the more accurate the prediction.
I do know that they are approximations, but have never found a case where it was the opposite of the predicted magnitude (of course I wasn't looking for it) The rocket body I am most familiar with is the cosmos, and have found heavens above predictions quite accurate. I am really interested in what others have to say.