I think you want to allow for the springs to lose arch so maybe make the shackles a 0.5” or 1” shorter than the above number from equation. Also the above equation sounds similar to how ive heard of going about this. Measure eye to eye of unloaded leaf pack. Then measure shackle mounting bolt hole to front leaf hanger mounting bolt. The difference is max shackle length so that the pack does not bind at full droop.. then like i said go a little shorter since the pack will lose arch.. Also a longer shackle will decrease negative arch with more bump travel since its taller.. But you need to get those drop hangers bolted up first.. also I read on here that your shackle should never point lower than your front hanger bolt when setting up droop/limit straps.. not sure the reason behind that though.