I'm in my third 50 Art and 5th Art lens overall. This current 50 is the only one without the inconsistent focus, and it's bought most recently. I bought the docking, but what I have learned is that when you get a working Art lens, the docking is absolutely not needed. My current 50 missed by the same at EVERY distance , so I set the afma in camera. And that was what other said when I had my problems, "I have a working copy, no afma or docking needed".
If it's not the same afma value at every distance you will never get it to work.
Buy the docking only if a new firmware comes out, if you're buying the 50, no need.