The comments about spine issues is with carbon. I have yet to see any issues raised with aluminum or wood. If I went 20# under spine with aluminum, I would have a wet noodle for an arrow. I think the calculator is on when only looking at spine deflection number as they relate to other materials. However, from what I have seen, carbon reacts so fast it can have issues clearing the bow, and will not plane of line as much as other materials. So I believe the dynamic spine of carbon is different than the measured deflection spine.