First strike does not mean one strike, which is the principle of delivering one quick and powerful strike in the hope that it will end the confrontation. Even if you believe in the one strike principle, it has no place in a self-defense situation where your life is in jeopardy. When you are defending your life, you should not waste time "feeling out" the attacker or counting on one strike to stop the attack. When you strike, it is with a flurry of quick, powerful, accurate, strikes that overwhelm the attacker.
If you wait for the attacker to strike first, you start out on the defensive. If your defense fails to stop the attack completely, you may be injured and unable to mount an offense. By attacking first, you are acting rather than reacting. You are controlling the situation and putting the attacker on the defense, which is not what the attacker wanted, so the attacker will probably seek the fastest way out of the situation. When an attack is inevitable and imminent, do not be timid or sorry about any legal ramifications. Just do what you have to do, or die.
First strike does not mean you may attack with impunity. You must never use force against another person unless it is absolutely justified and the force used must only the amount of force that a reasonable person under the same circumstances would have used to suppress the attacker. The decision to launch a preemptive strike must always be a last resort, where all other means of avoiding and defusing violence have been exhausted. Even then, you run the risk of having a criminal complaint or a civil suit filed against you.
So when may you reasonably think that an attack is imminent and reasonable defend yourself? Reasonability depends on two factors: the environment and the circumstances.