If you are to cook with a rice cooker, take the starting ratio of rice to water 10:11 by volume, adjust base on other factors. The one knuckle rule works becaus

If you are to cook with a rice cooker, take the starting ratio of rice to water 10:11 by volume, adjust base on other factors. The one knuckle rule works because in most families, the environment of cooking the rice, the tool, the people to serve, the type of rice, is more stable. The heuristics of one knuckle fail instantly if you want to cook for one person (too much water), or cook in open fire where water evaporates