When you cook larger quantities of rice (say, more than three cups dry rice), you need proportionally less water.  Can you provide a rule of thumb or a chart?  I cook both white and brown rice.
-- Marcia

I can find no detailed reference for the amount of water needed to cook larger quantities of rice but I notice that the instructions for my Aroma 20-Cup Digital Rice Cooker and Food Steamer shows decreasing ratios.  The table they give is in "rice industry standard cups" (180 ml) which are not the same as Imperial or US cups, so I have converted everything to US and Metric in the table below. 

Note that this is for rice that has been rinsed and drained.  This means there is some water adhering to the rice which is not shown in the above table.  The table provided with my rice cooker uses the same amount of water for both white and brown rice.

For really large quantities, at the top end of the table above, you may want to consider cooking your rice in a foil covered roasting pan.  To do that, preheat your oven at 350°F (177°C).  Once the oven is hot, rinse and drain the rice and place it in the roasting pan, boil the necessary amount of water, salt it as desired, and pour over the rice.  Cover the pan tightly with foil and bake for 30 minutes for white rice, and about an hour and thirty minutes for brown.  After that time, check the rice. If necessary cook longer, adding more boiling water, if needed.  The rice can sit for a while before serving, so there is no harm in starting to cook it a little early.  As always, fluff the rice with a fork before serving to loosen the individual kernels.

Note however that cooked rice left at room temperature provides an ideal medium for the growth of the bacteria Bacillus Cereus, one form of which lives on uncooked rice and both the bacteria and the toxin it produces can survive boiling temperatures.  This bacteria can cause food poisoning.  The bottom line is that cooked rice should be consumed or refrigerated within one hour of cooking.  Rice held for service should be kept at temperatures above 140 °F (60 °C).  For further details, please read the article Refrigerate Cooked and Leftover Rice from Colorado Sate University.

Also note that the above table is for a specific appliance so you will likely need to experiment a bit before you have the best recipe for your use.