Using distance driver to keep the control in the middle

Hi everyone,
I am trying advance in limb rigging by referencing rain rig. I currently stuck at the last step where toon elbow control stays in the middle of the shoulder and wrist. It is based on the distance driver and I would like to know more about the math it involves. I found following expression in distance driver driving transformation constraint.
max( 0, (distance-0.416*scale ) * (1/scale) /2)
Can anyone please tell where did 0.416 come from?