You wouldn't think this would be so hard. From now on, I'll just remember the cicadas in the following link:
http://www.learner.org/courses/learningmath/measurement/session2/part_b/
Also, this treatise was so well written that even if it didn't answer my question there's a lot to learn from it:
http://www.markcad.com/autocad/drawingscale.htm
