One question though, my bottles are marked 18mg, does that mean 18mg per 10ml then? Should this not rather be marked 1.8mg/ml or as a percentage so that the nicotine content is clear to everybody?
Nicotine in e-liquid is measured at mg/ml, but the /ml part is dropped when printed, mainly because it's shorter, and most e-liquid labels don't have a lot of space. If your bottle says 18mg, then it's 18mg/ml, not 18mg total.
Some companies do use different ways to mark their nic level, most common of those is %, where 1% = 10mg/ml. Another one that is mainly used by Twisp is mg/drop, which equates to around 1/20 of a milliliter, so 0.9mg/drop = 18mg/ml
Source: @Silver