TheZeroPointPrinciple is a mental evaluation tool that I use to make sure that I don't get angry when a relationship ends. Its base presumption is that anger at the end of a relationship is caused by a perceived inequity. Therefore, to avoid such anger, apply TheZeroPointPrinciple... in other words, at all times make sure that you have a BalancedRelationship? with the other party (lover, employer, dog, whatever).
Just take some time occasionally to step back and ask yourself, if my (employer were to fire me / lover were to leave me / dog got hit by a car) would I feel that I was cheated?
This sounds a lot like "If someone is using/abusing you, make sure you get them back by doing the same before you end the relationship." To me, this doesn't look like a sound principle of personal interaction.
This is a pessimistic view of it. The positive view is that it enables thinking (and then e.g. talking) about real or perceived imbalances early.