Divide your flow (GPM) by the number of nozzles on the SC'er (I assume 2). If you have a 5.5 GPM machine then you would look at a nozzle chart http://www.pressurewashoutlet.com/files/NozzleChart.pdf and find the 2.5 GPM line, run your finger over it to the desired pressure, and that is the size of both nozzles. (i.e. 5.5 / 2 = 2.75 [use 2.5 line] so 3000 PSI uses size 3.0 nozzles.