I'm one of those Licensed professional engineers mentioned above. But I'm not about to dust off some old text and work a solution for something I have not looked at since college. The handy servo torque calculator is just the tool. But don't forget about friction. If you have pushrods that don't move freely, that's additive drag. I know that's not supposed to be, but I've had some that were pretty stiff when I made them go whee I needed them.