Speed has also got to have some thing to do with it. It would make sense that a heavier arrow would effect spine.

I don't know how they come up with spine charts but If I did oneI'd err on the side of stiff/heavier just for saftey, then again I think GT's are the opposite to get the shaft lighter (their sales pitch).

They sell arrows according to poundage and supposed speed/cam, a heavier arrow effects speed. If speed is a factor then picking the right spine should include that not just what cam you have.