Just retested on smaller humanoid mobs at Castrum Centri. The angle is definitely less than 120 and the direction you are facing doesn't seem to impact whether you block or parry or how often you block or parry.

Of course, this isn't truly a "test" by any means, as I didn't count anything and compare data, and is just as anecdotal as Petite's friend, but I'm just sharing what I found so there is less confusion. Hope this helps!