Alrighty,

The selection of an arrow depends on a number of variables.

Draw weight.

Spine of the shaft.

Draw length.

Arrow length.

Shaft diameter.

Point weight.

Arrow Weight.

Centreshot of the bow.

Release (form) of the archer.

And so it goes...

You need to integrate all of these factors in order to get even an approximate idea of which spine arrow to use.

In addition you also need to consider the recovery speed and inertia of the various materials used to make arrow shafts. Carbon does NOT behave like wood, which does not behave like aluminium.

Silly little example- my CM Thunderbird (47#"28) loves 40/45 5/16" cedar shafts with 100gn points. But stick a 125 gn point on that or a 45-50 5/16" POC shaft, and we're going downhill.

I could tune the bow to the shaft, but considering the fact that I see nowt but feather on the favoured setup, I'll stay where I am for now.

Point is, you can't predict which arrow will work best for a particular bow without seeing all three in action.

Bear this in mind next time you advise someone on arrow selection.