To prepare the perfect dosas, use 3: 1 ratio of dal and rice. You have to soak 3 cups of idli rice and 1 cup of skinned urad daal in a large bowl of water separately for overnight. You can also add 2 tsp of methi powder to the soaked daal, which will help it in fermenting and will make crispy dosas. You can prepare crisp and thin dosas using this measurement.
Remember to wash the dal and rice thrice in the running water, soak it overnight and grind it in the morning. Cover the dosa batter with a lid for 5 to 6 hours, and it will be fermented well. You can further add salt according to the taste before preparing. Consistency must be not too thick and not too thin.
Note: You can also soak idli rice and urad daal in the morning, and grind at night. Leave the batter overnight to get fermented, and prepare dosas in the morning.