I know Beman and Easton size their shafts by the amount of deflection the shaft has, when a specific weight is set on the shaft that's supported at 28" between centers. A size 400 shaft deflects .400", a 340 deflects .340" and so on. The lower the number, the stiffer the shaft. Gold Tip sizes their shaft by the spine range they intend them for: 35-55 pounds is a 3555 shaft, 55-75 pounds is a 5575 shaft. Then some of 'em... who know WHAT they were thinking for their sizes. []

Too bad ATA hasn't stepped in and mandated some kind of universal sizing method.