I installed this shelf beneath a 4x8 shelf.  Measured 20" bottom to bottom on wall as instructed.  Positioned cables in the holes designated 20", and suspended with shipped hardware from the underside of the 4x8 in holes provided.  

The front of the shelf hangs 1/4" lower than the backside of the shelf.  

Now I will have to jetty-rig some sort of spacer to take up the extra 1/4" of cable length.  Not something I wanted to have to do with such an expensive engineered shelf.