Got it! Thanks for the extensive reply beresford, it really helps. So if I want to use Final Op fire against an enemy vehicle which is right behind me, I should:
-2 Op Fire
-1 Final Op Fire
-1 Firing after turning in hex
(no distance/smoke/elevation modifiers)
If I pass, I can turn 180, shoot and maintain my new position.
If I fail, I don't turn, I can't shoot and my enemy can attack me from behind.