From what I remember,
When I sold Lux units - The numbers on the dial represented the amount of air flow lost when the auto system kicks in. In other words, the 1 = 10% air flow loss 2 = 20% and so on...
Keep in mind, a quarter cup of flour can clog a bag beyond the 10% mark within seconds, while five pounds of sand (a filtering agent in it's own right) will barely diminish the air flow at all. It's the combination of fine dust and course "dirt" that comprises said "bricks".
The rice is supposed to dislodge clogging particles from the inner (genuine Lux bags) filter linings, allowing them to coalesce, thereby maintaining airflow longer than if not used at all.
It is imperative that the rice be Mahatma long grain yellow rice, uncooked of course - and no more than 134 individual grains.
(Disclaimer: the above sentence is an attempt at humor and should not be considered a fact)
Frankly, I don't pay much attention to the automatic feature. I do have mine set at 2.5 (or thereabouts) but rely on the sound of the motor and sound at the nozzle when deciding to change the bag.
The automatic feature is cool to play with though - especially on a LX!