Get and stick in the draft :-)It seems it's average over the race, but like you say it's impossible to know what your average is until you finish. I was certainly way over 3.2 at times, so was having to knock it back to ensure the ave was below 3.2 which I only just managed. I think there must be a trick to it ... maybe power up the hills and hold good power on the flats then drop the power on the descents ... maybe one of the regulars can advise?
I'm not quite to the point where I need to watch it closely, but when I get there, I'll just have to remember to turn on the garmin at race start and check it near the 20 minute mark.