My Athena rear brake is annoying the crap out of me and I can't figure out how to center it.
It's simple and the usual procedure for centring single pivot brakes. You need to get a spanner onto the flats on the main bolt through the brake. This will let you rotate the brake body against the friction of the nut holding the brake onto the bike, so that nut can't be too tight. Squeeze the brake lever a few times, see how the brake moves, and use the spanner to rotate it one way or the other so the brake calipers are nicely centred. If you backed off the brake nut a little to allow this to be easy, now squeeze the brake levers to close the brake and tighten the brake bolt again.
The length of the brake outer from the cable stop to the rear brake can affect the brake centring although I have found the Campag single pivot brake to be not oversensitive about this.