These are the cooking instructions from the side of a box of brown rice:

1. Combine in a two-quart saucepan: 1 c. rice 2 c. water 1 Tbsp. oil or margarine, optional

2. Stir lightly; bring to a roiling boil and reduce heat to simmer (low boil).

3. Cover with a tight-fitting lid and simmer for 35-45 minutes.

You are correct about the increase to 1 1/2 cups of rice. But generally speaking, when you steam whole grains if you increase the amount of grains, there DOES come a point where you have to use a little less water. That point is usually the 2 cup mark.

So if you use 2 1/2 cups of brown rice, instead of doubling the water to 5 cups, you would use 4 3/4 or 4 1/2 cups water. If you go to 3 cups of brown rice, instead of using 6 cups of water you would use 5 1/2 or 5 1/4 cups water.

So you see, as the grains go up in increments, you use slightly less water each time. And there is also some trial and error depending on if you are at sea level or in the mountains, etc.

Mostly, you just experiment to find what works for you. But this should be a good starting point. :)