Its pretty much trial and error, though after a short while, you do get a pretty good feel for how many rotations of the knob you need to make to change x seconds of flow. I can even go from normal beans to decaf in 1 or at most 2 trials.
What I found very helpful was putting in your own gauge or zeroing point as you mentioned. You will find a photo of this in my Iberital Challenge review here: http://coffeesnobs.com.au/YaBB.pl?num=1183623068
Without the markings, it is really quite hard to set a baseline for further adjustments. Hope this helps!