As a RDM main I have learned that any part of the target's hit box has to be within a certain arc in front of your character to successfully cast. You don't even have to look at the center of it's hit box, as long as a portion is within your character's "sight arc" the cast will go through. This is very useful for uptime on gaze mechanics since those use the "Center Point" of the enemy's hit box and your character's sight arc to determine if you get hit by the gaze mechanic.