I remember our Japanese cook measuring and cooking rice using this technique as a child. My sister-in-law is married to a Japanese and she learned this technique as a bride when she went to Hawaii to live.
1Put the palm of your hand flat on the surface of the rice and add water until the water touches the knuckle of your middle finger. You now have enough water to cook your rice.
I have no idea why, but this method has worked with every size of rice cooker I've ever used, and every (adult) size of hand I've ever seen... it works when I just wash whatever rice is left in the bag, toss it in the pot, and fill. It's also great for sanity checking when you can't remember whether you washed three or four cups of rice or what.
2Cover pan or rice cooker. If cooking on stove bring to a boil and then lower heat to lowest setting possible. Continue cooking for 20 minutes. Fluff rice before serving.