Unless you have quite the background experience around rebuildables (which is the collective knowledge regarding ohm's law, battery safety, anti-short measures, what to do in case of disasters, airflow, etc) you should never go to extreme sub ohms such as .3 ohms. While it might not seem so dangerous just by reading our warnings, the wrong battery and that super low ohms is a recipe for a blown mod and body damage.
Most of the info has already been covered. I've gone down until 0.2 ohms on an RDA. Granted it was an experiment and I cannot vape that, but I got to that point after building hundreds upon hundreds of coils in different setups and reading a lot and listening to more experienced vapers. My tips are, start off at a 1-1.5 ohm range until you really get it down and understand everything about it. Have a multimeter at all times and be sure to check for any sign of shorts before firing your freshly made coil. Your battery is extremely important. Whenever we are talking about the hardest hitting, less voltage drop, safe batteries there's two that come to my mind. The Sony 30 Amps and the AW IMR, both in 18650. For really low sub ohms I have come to only trust the Sony 30 Amps, with the AW IMR coming in second.
But as it has been said previously, start from that 1-1.5 ohm range. You'll eventually find your sweet spot and it might not even be sub ohms. I'd highly recommend learning about micro coils for a higher quality vape. And for the love of God and your face, do not ever fire that range of sub ohms on the current batteries you own ever again.