The formula is much more complicated than I am willing to figure out myself.

The arrow spines are calculated through on target software and are shown on a moving graph to dial in the optimum dynamic spine. Throw on a heavier tip and the pile weight increases on the aroow, effecting the spine greatly. Put on a lighter tip and the pile weight decreases.

I have used on target software for getting the exact dynamic spine match for my setup. It may require shooting a pound or two heavier or lighter than I wanted, but having the arrow dialed in makes a noticible difference.