Well, the good news is that you are selling less than expected doses...
Here is the general math involved...
Cigarettes "CAN" contain between 6mg to 30mg, depending on the plant and processing of the tobacco.
Of that 6 - 30mg within a cigarette, it is common that, with heat, combustion, filtering, and wasted unsmoked burn-away, that only 1.2mg - 4.8mg is consumed. (4.8mg for unfiltered cigar-like cigs. 1.2mg - 1.8mg for most common cigs with a fiberglass filter.)
Untreated tobacco is used for extraction, not chemically treated cigarettes. The paper-treatment, and tobacco treatment has undesired chemicals that contaminate the extracted nicotine. When/if you get nicotine crystals to form, they will have trapped some of these impurities within the crystal structure. (Though, the toxicity will be greatly reduced, when compared to the same toxin levels that are in a cigarette it was extracted from.)
Unless you have the extracted solution, or mixed solution tested... you should ASSUME that you have the highest possible concentration, not the average concentration. (Average is around 20mg, while high would be 30mg per cigarette.)
If you have extracted, or created pure synthetic nicotine, you can just weigh the crystal powder. Measuring liquid requires a reagent test, and/or spectrum testing.
When manufacturing, you would create a large batch of extract, seed it, and remove only the crystals. The crystals would be dissolved into a refinement solution that helps to chemically draw-out impurities with ionization, and/or physical microbial digestion to a product that can be skimmed or evaporated, or left within the solution.
This would be seeded again, and pure, or near-pure crystals would be removed, tested for purity, and held until the evaluation came back. The evaluation would tell them how to use the extraction, for mixing, since the concentration is now known.
You MUST assume you have more nicotine than you could possibly have, because if you assume and publish that you have less, but you actually have more... that places you into a more dangerous undesired legal position of selling poison, falsely represented as non-poisonous levels. As opposed to the small legal battle you would face, if you sold less, as more... Selling it as 34mg, but it actually being 20mg, is not as bad as selling it as 34mg, but it actually being 60mg.
As long as you state that the solution you sell is not for direct undiluted use, and the concentration of the marked solution is equal to, or less than the marked value. You should be mostly safe.
There is a draw-back to creating low-nicotine solution...
Your body desires a certain level of nicotine, based on your prior habits. Giving less nicotine, translates into the desire to toke more VG or PG and flavoring. This could create more health issues, if the person has a reaction to high levels of VG and PG and flavoring.
On the opposite side... (Like the way I toke)
I use higher levels of nicotine, so that I take-in less VG and PG and flavoring. I am satisfied with my 4 tokes of "High Nic", as opposed to the 20 tokes of "Low Nic", that I would need to get the same level of nicotine. I don't have time, or battery power to waste toking away all day long. I need a quick short blast now... one five minutes later... another five minutes later... and if needed... one more five minutes later.
However... the latter use, is NOT for a NOOB, who will attempt to toke 20 hits for five solid minutes. That will make them sick, or trigger some serious condition, before they have time to stop... EG... It takes a minute or two before it is all absorbed, and by then, there is no way to remove the high level they just consumed. Only in distant tokes, can you "Feel" and regulate intake.
So...
You could also add a solid disclaimer...
This extraction is for non-human testing only. By purchasing this unregulated concentration, you are agreeing that you will not sell, consume, or hold (______) liable for any damages related to the abuse of the untested materials provided.
Though, I would still consult a lawyer.
The other alternative you could offer... As opposed to selling extractions... You could sell extracting-services. They would provide the desired tobacco, and you would extract the physical plant material, and reduce the solution into the lowest possible purity that you can. (This would still require individual batch testing, so you could dilute the solution into a less dangerous concentration that is within acceptable limits. I believe the acceptable limit is around 40mg per ml for concentrated solution.)
However, there is also the issue of...
What are you diluting it with?
Are you using pharmaceutical grade certified chemicals? Can you prove it with records of purchases and tested, registered batch numbers? Are you operating in a clean-room? (A clean-room is not a room that is scrubbed well. It is a sealed room that has a positive-pressure environment, maintained by a medical-grade hepa-filter system that filters down to 99.999999% of 0.000001 micron particles and bacteria. While you operate in a clean-suit, wearing a full head-net and breather-mask. Filling only sterilized bottles which are then sealed while within the confines of the clean-room.)
You could always find a manufacture who accepts "Recipes", and will be willing to do the work for you. You just have to demand that they state they have a USP certified laboratory for production. (Or similar requirement. You need to consult a lawyer about that also, or at least a similar manufacture.)
Just some food for thought...