[Utah-astronomy] Predicting the brightness of a satellite at distance x, how to

I would like to make a rough mathematical estimate of the magnitude of a Centaur booster at various distances between 500 and 400,000 kilometers.  The object is 12 meters long by 2.5 meters in diameter.   The albedo is not known.  A best guesstimate of the albedo will have to make?  Can anyone provide a formula or reading references?  Thanks, Kurt  P.S. - I have already collected empirical examples of detections at various distances.