Altruistic acts and Acts of Supererogation

I have already defined an act to be altruistic if it done mainly to help others at little or no benefit to the helping person, and goes above a certain call of duty or expectation.  To perform an act of  supererogation is like to perform an act of heroism.  The main difference between the two (altruistic and  supererogation) is degree.  It would be altruistic to donate an additional twenty dollars to a charity.   Supererogation might call for hundreds or thousands of dollars.  Usually an act of  supererogation would involve great risk of physical harm.  Such acts are often done with thinking, just enough thought process to decide to do it.  Taking the risk to save someone from an oncoming train is heroic, and altruistic.  Helping the person get back up on the platform is altruistic, but not heroic.