Well according to the law you are only able to use "reasonable" violence in self defence or in defence of another person - but the definition of "reasonable" is wide open to interpretation.
Personally, I wouldn't be standing there debating with myself whether I might hurt someone "too much" if that person is attempting to harm me or someone I care about. Instinct would kick in and strangely enough I wouldn't be giving a toss about the attacker at that moment.
