It really isn't black magic. What it is is a very tough job by someone at CNATRA who is trying to make the best decisions based on good information, but also on predictions of the future. Like a weather forcast, these predictions can be way off and they can change from day to day.
I second the above. And NSS actually is pretty transparent if you take the time to really look into how its computed (most of its explained in the MPTS).
Granted, with flight grades there will always be some subjectivity on the part of the instructor. Show up prepared, act like you care, nail the brief and that subjectivity will act in your favor. Practice in the sims as much as you can. Work hard, take care of everything that is in your control, and dont worry about the rest.