The Odd One is right here, calculating the ACNs for aircraft seems to be an obscure art with weird formulas and is not that evident at all. I do not recall the details anymore, but I remember from a lecture at univesity that there was a problem with the 777 since it has three axes instead of the usual two on the main landing gears and that Boing did some rather questionable calculations to obtain a three-axes fromula.
Sorry, I do not remember any more details, only that there was a problem with 777 due to the different main gear configuration.