Remember, the lower the number, the stiffer the arrow. What spine you choose should be dictated by your bow's poundage. Some draw-weight settings allow you to choose between a range of spine offerings. If you're looking for maximum speed, go with a lesser-spined arrow that fits your draw-weight category and has a grain-per-inch rating that's still safe for your poundage but will make the arrow lighter.   If you're looking for a quiet, hard-hitting shaft that ensures deep penetration, go with a stiffer-spined arrow in your weight category. These arrows will also sport a higher GPI build. I pull 70-pounds of draw weight and shoot arrow spines of 340 and 350. My wife shoots a draw weight of 45 pounds and shoots a 400-spine shaft. If she is looking for a tad bit more speed, she will shoot a 500-spine arrow.