Those charts are based on 1.8-2.2 Ohm ratings and probably 6-8W of power.
Think of it this way... if you need 18mg at 2.0 Ohms, then when you lower the resistance by 50% to 1.0 Ohms, you need to lower you Nicotine level by at least 25%, up to 50%. Using a 0.5 Ohm coil would require an equivalent juice at 4.5-6.0mg, and depending on your vaping habits (i.e. Chain-vaping), you may want to use 3mg?
Heat is the factor here. Nicotine is absorbed exponentially faster as heat increases. The hotter your coil (lower resistance), the less Nicotine you need to vape to get the same effect as you do at higher resistances (cooler vape). This is a proven fact from tests done on absorption rates from the Nicotine patches. Raising the temp of the patch by 5-deg F, increased the absorption rate by as high as 20%, causing light-headiness, nausea, or, at minimum, the usual "Nicotine-rush" you use to get with the first cigarette in the mornings. So, as heat increases, you don't get MORE Nicotine... you get it FASTER, which is why you have to use lower Nicotine level juices when you sub-ohm!