// This might help you.
nullPointer = thisComp.layer(“your null name”).transform.position;
thisPsotion = transform.position;
// add a slider effect to the null.
falloff = thisComp.layer(“your null name”).effect(“Slider Control”)(“Slider”);
l = length(nullPointer,thisPsotion);
i = linear(l,0,falloff,100,0)
// just change the value of the slider control to get the result you looking for.