I've been told that the 'needle opening' only controls the 'width' of the actual paint-spray coming out of the tip. So if you open the needle more, you will get a wider spray, and if you close it, you will get a narrow spray. For me, the viscosity of the paint is (almost) everything. I very rarely change tips. But if the paint is thick, go to a bigger nozzle, and if the paint is too thin, go to finer or smaller nozzle. To check the viscosity, I mix it good and then tip the paint-jar so you can observe how the paint 'swirls'.