Question  about nozzle diameter and filament output.

Question about nozzle diameter and filament output.
Said, a 0.3mm nozzle doesn’t produce 0.3mm filament output, assuming using ABS, THEN setting of layer height have nothing to do with what a nozzle dictate but rather the actual output. Whatever theoretical calculation doesn’t really work, a 10% and 20%, or a 0.05 or 0.1mm doesn’t shown a distinctive differences. It actually depending upon what is extruded, factors such as material quality, extrusion speed, extrusion pressure, temperature set, smoothness of the barrel, nozzle assy,…etc. THEN a 0.3mm and 0.35mm or even 0.4mm nozzle is just a myth that will produce significant differences since it doesn’t related to actual output and confident of how accurate the nozzle made is questionable.
Anyone have any pointer to actually or have a best known method to tackle such issue??

Neither the slicer nor the printer really care much about the free-flowing die swell. There are two things that are relevant: 1) the extrusion width, which is a number based on the nozzle diameter and other factors. It is normally set about 20% higher than the actual nozzle diameter. Anything lower does usually not print well.
2) the volume the extruder pushes through the hotend. Everything that goes in comes out the other end. Along with the set layer thickness and the filament diameter, the slicer can calculate how much plastic to push into the hotend to get the desired extrusion width. There are long threads in forums and here on G+ that discuss the ideal way of calculating these.
The only real influence the size of the nozzle has is the ideal extrusion width. Smaller nozzles are often better suited for smaller layer heights and can reproduce smaller details on each layer, but increase the printing time.

Smaller nozzles allow thinner extrusion lines, and thus higher details, at the cost of maximum extrusion rate (measured in mm^3 per second) it’s that simple.
(There are also some constrains with nozzle shape and layer thickness, but those have much more variance)

Everything else is just software tweaks. The software defines how much extrusion it wants. How thick the layers it wants. And thus how width the lines are. I’ve seen people do 0.8mm width lines from a 0.4mm nozzle. And 0.2mm lines from a 0.4mm nozzle (with tricks)