To help find/match your current nicotine intake there are many things to take into account. Now, bear in mind, these are all guidlines (an analog manufacturer can not tell precisely how much nicotine is in a smoke, let alone us being exact about it!). Strengths being listed are mg/ml (18, 24, 36 etc). A milliliter is about 20 drops, I say "about" because even this is a guidline depending on viscosity of the fluid, ambient temperature, atmospheric pressure, and such.
Before I go too much further, I do have a background in medicine and chemistry, just trying to be helpful.
As far as nicotine in an analog smoke, your intake of the same cigarette will vary greatly compared to my intake. I smoke (did) Marlboro reds, 1.5 packs per day for 20 years. I inhale deeply, hold long, and smoke even part of the damn filter! I get maximum intake from an analog, I do not take much time between hits either. I did not smoke for the enjoyment (well maybe at first), but for the sole purpose of nicotine absorbsion. The general number I use for calculating nicotine is 1.2mg per analog. If you inhale deep and hold long it will be greater, if you do not inhale or hold at all it will be much less. Now, with this in mind, you also need to allow tolerances for pure volume distribution. Inhalation is the fastest means of absorbtion in the body, the contents of what you inhale with vapor and smoke are two different monsters. An analog cigarette has much more "junk" in it which diffuses the nicotine, so the concentration is much lower than vapor from juice (on an apples to apples comparison). Vaping is a MUCH more efficient means of nicotine absorbtion than smoking an analog. So, moving forward......
The combination of a crap-ton of variables in vaping makes each user unique. What works for you, may not work for someone else, do not get discouraged, it is a learning curve, I am just trying to lessen that curve. I do wholeheartedly believe anyone can give up analogs for vaping under the correct comination of variables. When I first started, I KNEW I was getting too much nicotine, but still wanted an analog! We'll approach that in a sec. You would most likely not find two users here that smoked analogs in the same manner (strength, flavor, inhalation depth, speed, etc etc), so you most likely will not find two vapor users of the same use habits.
There are many things you want to "match", or closely simulate, when converting from analogs. Let me give you what worked for me and that will give you an idea of the point I am getting at.
I calculate my analog intake at 1.5mg per smoke (most likely a touch less since smoking is less efficient than vaping), but this is a guideline! 1.5mg x 30 (pack and a half/day) is 45mg. Now, could I vape a mixture of 45mg/ml and vape just 1ml? Sure, but that would not cut it for me. You can vape 1ml pretty quickly. Let's use some round numbers and convert the 45mg/ml to 24mg/ml and use 2ml. I decreased strength and increased volume, yet the same (roughly) mg of nicotine. Now, using 24mg/ml juice and a volume of 2ml, this gives me roughly 40 drops (20 drops per ml) to vape in a day.
I have 40 drops of 24mg/ml. Now, matching the physical characteristics of smoking! This depends alot on your equipment. If you can replicate your smoking pattern that pre-existed and 40 drops lasted the day, great. If not, and you found yourself not vaping enough (not enough hits to match your old school style) you can cut the strength to a weaker concentration and just vape more often. If you find yourself sucking the thing all day and want to get rid of that, increase concentration and use it less.
This is a balance which needs to be found for your liking, not two people will be the same. You also need to account for "throat hit", this was my biggest obstacle at first. I needed more "harshness" or I did not feel my craving was fullfilled. You may need to take longer hits, or cut in a drop or two of alcohol, or any other suggestions on these forums to increase throat hit. It IS a factor in many peoples dependency on analogs.
For menthol smokers, flavor is a big factor. Many new people buy the pre-filled carts that just do not cut it, you need to experiment with menthol flavoring and strengths. The manafacturers ship cartridges with starter kits just to have something to ship with them. They most likely will not make the cut! The marketing statements of these carts are absurd also, one cart is NOT the equivelant to XX packs of analogs, for reasons mentioned above. One pack of Marlboro full flavor kings with my habit of smoking analogs gives me approximately 28mg of nicotine. Now, for a cart to hold a 1 pack equivelant it would have to hold 31 drops of 18mg/ml juice!!!! Ain't happenin! realistically, from the manufacturer, a cart packaged as high stength (say 18mg/ml) will hold 7-8 drops on a mini. You get 0.9mg per drop roughly, this would equate to that cart holding 6.3 to 7.2 mg of nicotine. The way I smoke, this is about 4-5 analogs worth (and I am erroring in favor of the cart manufacturer here, it is most likely less!).
So, you get a new starter kit, pick out a cart, and after a few hits you just don't feel satisfied.... It ain't you! It's just a combination that needs work! Happy vaping, hope this helps someone!