For you to legally defend yourself and counterattack, there should some action by the attacker that triggers your reactions. The trigger could be the attacker cocking an arm back for a punch, or it could be a challenge to your fence.
When you are in a high fence and the attacker has a weapon, the trigger is the weapon itself and you may act whenever an opening presents itself for you to control the weapon and counterattack.
When you are in any of the middle fences, if the attacker moves into contact with your fence, it indicates an immanent attack and should trigger your defenses and counterattacks.
When you are in a low fence, the trigger is an invisible vertical plane located about half way between you and the attacker. Once the attacker crosses this plane, it triggers your defenses and counterattacks. The range you maintain depends upon a number of factors, such as your size, power, and quickness in relation to those of the attacker, how much you think the attacker intents to harm you, and any other people in area, such as your family. You may adjust the range to change the location of the plane, or you may change the location of the plane without changing the range, such as by moving it closer to the attacker to give yourself more time to react, or by moving it closer to you to help prove that the attacker was actually trying to harm you.