Bill Welch wrote:
"Some of us have no idea who Boomer Pete is/was."
Boomer Pete was a pen name that Al Kalmbach used to author various articles in Model Railroader. The specific reference here is a piece written by him detailing the mix of home and foreign road freight cars. He logically stated the mix should be a 50-50 mix of home road and foreign road cars, with foreign road cars weighed favoring proximity to the home road. This idea gained unwarranted legitimacy as it was repeated in of John Armstrong's "Track Planning for Realistic Operations" (still worth getting an early edition, as later editions dropped a lot of material, and both are superior to what Kalmbach is currently offering with regards to books on model railroad operation).
The problem is that depending on the type of freight cars, this isn't the case, especially with general service cars as plain boxcars and flat cars, which tend to average out to national fleet averages. For extensive e-mails on the subject, do a search on the late Tim Gilbert's e-mails in the group archive. Or for a nice summary, see Tony Thompson's blog entries on the subject.