Here is a link to a post I made at OCAU for working out the target flow rate of your system given a few things that you can figure out for yourself.
As Bill says, getting to more than a 10% accuracy is hard, especially when hand-plotting/reading information from crappy PQ graphs/plots, but for the most part, 10% slop is "good enough" unless you're down below the 2lpm mark.
Thread is here:
On pressure drops and flow rates