Keep in mind that there is more to what line weight a given rod blank will cast properly than the diameter of the "butt". The amount of graphite in the blank, the type of graphite used, the type and amount of material used to provide 'hoop strength' to the blank. the way the graphite was cut before being wrapped on the mandrel. etc.
It is not simply a matter of taking a tip or top 2 sections of a 4-piece 15 ft rod and seeing that the diameter of the rod is .300 or .285 or whatever that determines what line it will cast properly.