Millets have a higher satiety index when compared to white paddy rice, i.e. one feels full and sated after eating a smaller quantity of millets as compared to white paddy rice. As one would expect the satiety index is not a very objective quantity. It is a statistical construct to quantify a subjective thing and hence should typically be considered as a range or as a comparative measure rather than an indicator of an absolute characteristic.
The satiety index for millets, generally speaking, is about 130 to 150% as that of white paddy rice. i.e. one would need to eat 1.3 to 1.5 cups of cooked white paddy rice to feel as full as one does after eating 1 cup of cooked millet rice. Little millet rice would be on the lower bound of this range while finger millet or pearl millet would be at the upper bound of this range.
With this in mind, let us now get down to planning a meal in which we will prepare a millet rice instead of white paddy rice.
Remember that white paddy rice typically needs 2 cups of water for each cup of dry rice measure, while 1 cup of a dry millet rice needs about 3 cups of water to cook. For some millet varieties this can be even up to 3.5 to 4 cups of water. The best way to identify the exact proportion is to prepare a small quantity and verify.
Now lets say that a family usually cooks 1 cup of white paddy rice for their meal. So this would mean that typically eat about 3 cups of cooked white paddy rice. Applying the satiety index proportion of 130%, we get 3/1.3 i.e. approximately 2.3 cups of cooked millet rice. Using a 1:3 proportion of water, we would need to use about 0.6 cups of dry millet rice and 1.8 cups of water.
Hope this helps ! Feel free to point out errors, ask clarifications …