IMHO, don't beat yourself up if you sneak a smoke now and again. Just don't make a habit of it. We have a saying here on the forum:
It's not how many smokes you had that's important. It's how many you DIDN'T smoke that is.
How many cigarettes a day were you smoking before? 18 mg is a good place to start for a pack to pack and a half a day. I think you cut down the strength way too fast. It takes most people a year of vaping to go from 18 to 6 mg in my experience. Dropping too quickly usually leads to failure.
For e-liquid, the typical nicotine recommendations are generally determined by how much you smoked, and are as follows:
1/2 pack per day or less = 6 mg nicotine
1 ppd = 12 mg
1 1/2 ppd = 18 mg
2 ppd = 24 mg
Should you choose to use a sub-ohm juice attachment (coil less than 1.0 ohm); drop the above recommendations by half.
Figure on consuming at least 3 - 5 ml of liquid daily. Bottles are usually 10 or 30 ml volume.
E-liquid generally has concentrated liquid nicotine, propylene glycol (Pg), vegetable glycerin (Vg) and flavoring extracts.
PG and VG are usually used together in different ratios in e-liquid. For example, a 50/50 PG/VG ratio is common. Some juice vendors offer a range of different ratios depending upon customer preference.
PG = Propylene Glycol - the "flavor carrier" of e-liquid. A secondary source of "throat hit", but not as strong as the nicotine concentration. Some people have a sensitivity or even an allergy to PG. It's very thin in consistancy.
VG = Vegetable Glycerin - the "vapor maker" of e-liquid. Used as a sweeter in many food sources, and also as a skin emollient. It's very thick in consistancy.
Sweeter (sugary) or darker flavors tend to clog coils and wicks faster. E-liquids heavy with Vg are thicker in viscosity and may not wick as well as thinner e-liquids in some tanks.
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Good Starter Setups for a Beginner Vaper