A little forum search might have helped
Persuasion hinges on three things:
A: Your persuasion skill
B: The susceptibility of the NPC to specific avenues of persuasion
C: A bonus derived from your attributes.
A and B are essential. C is a bonus that can sometimes help you out. Don't count on it.
This means that you should think about how the NPC will react. Threatening a magister in the middle of his HQ is not going to work, however high C is, and even with high persuasion skill it may fail simply because the magister doesn't think that your threat is credible. He might, however, be easily swayed if you fawn over him and tell him he's the greatest and you're just there to touch the hem of his robe.