Comment by Omin

The fixed ratio doesn't scale all that well, because some of the water is absorbed by the rice and some just evaporates. Even with a lid on water still evaporates. The evaporation portion depends on the type of pot, how long you cook and how much heat you put in.

So, if you only vary amount of rice, you really need some_factor * amount_of_rice + constant_amount and the knuckle rule has that characteristic.