Ashbow,
It seems we are on the same track. Stiffness is measured with arrows suspended between centres as a deflection. This is then applied to the model of a standard arrow and said to be the spine of the shaft. This is only true of the arrow thought of in the model. If the arrow is cut shorter than standard it is stiffer and its spine goes up. If the bow is heavier that in the standard model, the stiffness of the arrow is the same but the spine goes down.
What this means is that stiffness is the measurement and spine is a quality taking into account the environment (the bow weight, brace height, the speed of return of the limbs etc.). Once you have tested a certain type of arrow on your bow, you can relate that to spine charts and work out whether an arrow will perform well on your bow or not.