Yea the adjustment is all in the mix, and the number of bottles, and its not a perfect science by any means. Mine certainly produces more at first, and then tapers off slowly until the point in which I refill it. I suppose you could run a valve on the line going into the tank, so that you could adjust it to blow off instead, to have the option to turn it on and off as to how much co2 you need, but I have found easiest is adjusting the level of co2 you need by adding or subtracting additional bottles of yeast/sugar. Some people end up running 3 2liter bottles simply because that is when they achieve the amount of co2 desired.
Pressurized is more of a perfect science, where you can really hone your system to the perfect amount of co2, but I like the idea of all natural! If only you could figure out a way to sustain the yeast indefinitely for a never ending co2 reactor ;p