Yes your graph is exactly what i described.
For getting left\right direction:
- add CROSS product of forward and that direction to actor vector.
- check its result Z value, if its positive your angle is to the right, if Z is negative angle is to the left
- or simply get SIGN out of Z value from CROSS product result and multiply that sgin result by degrees from arc cosine.
No worries, i have friend which is doing game with me. And he is scared of whole vector math. Or rather quite confused by it. Its common to scratch head when that is involved. Luckily or not i was forced to learn magnetic field theory which is full of vectors, that is why i remember dot and cross products.