These spots are already represented clearly in game:



Anything outside of those two small circles is irrelevant. If you're not allowed to hit the target from the rear (i.e., because of directional parry/counter), stand in the little yellow Flank section. If you're not allowed to hit the target from the flank, stand in the red Rear section.

If you're hitting it from the front, it's because you're not allowed to hit it from the flank or the rear, in which case just stand on the Facing pointer with the rest of the party.