The 3 digit static spine number is how much (in thousandths of an inch) the arrow shaft bends when a 1.94 lb weight is suspended from the middle of a 28” section of shaft. This spine rating methodology is consistent across manufacturers, so a 350 spine Bloodsport shaft will bend 0.350” during the static spine test just like a 350 spine Victory shaft. However, the methodology each manufacturer uses to recommend the spine you need for your particular bow/arrow setup can vary.
Spine selection is primarily a function of draw weight, arrow length, and the amount of weight on the front end of the arrow. Bow speed, let-off percentage, and rear end arrow weight also affect spine selection (to a lesser degree). Some arrow manufacturers (such as Bloodsport) “dumb down” the spine selection process and base their recommendation only on draw weight and arrow length. The unstated assumption in such simplified recommendations is that you’re putting a light-to-moderate amount of weight on the front end of the arrow (i.e., 100 gr broadhead and 20-ish gr insert). If you’re putting more than a “normal” amount of weight on the front end of your arrow, you may need to choose a stiffer shaft (smaller 3 digit spine number). The better spine charts/calculators (such as Victory’s), account for varying amounts of front end weight. The best spine calculators (i.e., Archers Advantage and Pinwheel) account for every detail of your bow/arrow setup.