Interesting topic.
IMO, the average of a line is a concept that cannot be adequately defined. It is a "general idea" of what can be expected from dogs of that line, not an absolute. However, IMO, general knowledge of the average of the line (ultimately) is a far more important understanding to have than merely knowing who "the best" of the line is ... if a person is looking to invest his time and life in breeding dogs down from that line.
For example, if "Slick Rick's Grand Champion King Kong" was the greatest dog from X line ... and one of the greatest dogs ever ... and you decided to base your yard on this line because of this ... but the AVERAGE of X line consisted of substandard animals (with King Kong just being a freak exception) ... then the lion's share of
your efforts would be wasted in feeding & breeding substandard plugs and producing something generally unable to compete with your peers.
By contrast, if you happened to like "Dogman's Ch Spot" ... who, though he may not be "the greatest" at anything, was still
damned good at everything ... and if the average of
his line was known to be damned good, consistently-competitive dogs "in general" ... I can truthfully promise you that you would be FAR better served linebreeding off of Spot's line than linebreeding off of King Kong's line (even if King Kong were the better dog).
Therefore, although not able to be quantified exactly, a basic grasp of the true quality of "the average of a line" is a FAR more important understanding to have as a dogman than is knowing that some particular dog was "the best" of his line
Jack