First step is get a decent ohm meter that at least reads to the hundredth decimal point and know the tolerance of that meter. Some of the basic meters sold by
vape websites have a tolerance of 0.05 or greater and that makes a huge difference in power output when dealing with low resistance
coils.
As to how low you can go I would say 0.2, however that should not be your goal. You just want to make sure not to go below that point.