How do you know when the batter is fermented enough but not too much? Is it to taste/preference?
When the dosa batter gets fermented, it doubles in volume and aereates well. To know if the dosa batter is over fermented, just taste a small part, it turns out sour. To prepare dosa batter, be sure the temperature is not too hot. If it is, it won't take much time to ferment the batter (5-6 hours). Refrigerate the batter so that it does not get over fermented.