If I remember correctly, the stated arrow spines are at 29 inches. So that means if you go shorter the spine is greater as there is less room to flex. If you go longer, the spine decreases. I will sometimes shoot a longer than needed arrow to get the arrow to spine out just right for my setup.

My spine is around a 440 and I can obtain that by playing around with the tip weights and the arrow length, all while trying to keep the desired FOC.

I found it alot cheaper to purchase an archery program which allows me to try different adjustments to my arrow and see the desired effects long before I even cut an arrow. The cost of the program that I purchased is alot cheaper than a dozen useless arrows......