I keep telling people..."You don't want to know how I mix mine"...but then I tell them how I do it!
So if I want about 10mg/ml nic, and I'm using 100mg/ml nic base, I know that I need to cut that down to 1/10th strength.
So I pour what looks like about 1/10 the bottle volume of nic base into the bottle, then top it off with what looks like the remaining 9/10ths of the bottle volume** with the cut solution. Shake the hell out of it, and vape it. If I think it's too weak or too strong, I adjust it, but I'm OK with it 99% of the time.
If I want 20 mg/ml, I just start with double the nic base.
I have a syringe and a "tube" with measurements around here somewhere collecting dust....
You know, I don't think they made cigs that had an accurate amount of nic in them, they varied slightly per cig. In later years, they reformulated and made them more uniform, but...this isn't rocket science. Just use your brain when playing with 100mg and don't get stupid.
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** accounting for leaving an air bubble for shaking.