Sorry, I am not completely sure. But here is my experience: In years with AGE games I have never had large NM gains/losses due to surrenders. I am not even sure if I have ever seen more than one point in NM. In particular, I remember feeling badly cheated, when I experienced for the first time how three enemy corps capitualted in a city without any significant NM gain for me.
Therefore, I am pretty certain that the amount is calculated independantly of the number of hitpoints that are lost.
Most likely it is a fixed amount (1 point). Or perhaps it depends on the size of the city???