Most arrow companies will have a spine chart for their arrows.

Just look up your draw lenght and weight, where the 2 colums meet will have what spine arrow you need.

Easton and Gold Tip have calucators on their web sites I am not sure about the others tho but they should.

If the charts say you are border line with say a 340 spine then get the next spine up which would be a 300 spine, you are better off being a little on the stiff side than the weak side.