I have an axis aligned box, with a 3D point ("center") inside (or even outside perhaps) the box. What would I like to do is calculate falloff from this point to an arbitrary position inside the box. If the box was a sphere and the point would always the sphere center, this would be pretty straight forward, but since the "center" point can be moved... I'm not looking for all out precise calculations, an approximation would suffice (speed wise). Currently only linear and cubic falloff are needed. Any ideas of how to handle this?

You are right... I'm just gonna use a sphere, simpler and faster. I'm working on a deflector/attractor system for particles. Don't really know why I wanted to complicate with boxes and arbitrary center point...