Minor correction
We allow a max strength of 36mg to be advertised here on ECF, vendors can sell what they like on their sites, for retail strengths (but not pure nic as the risk is too high for people who don't realise that it's as dangerous as opening a bottle of cyanide in their house, it needs a fume cupboard).
At some point we might move up to a max strength of 45mg allowed to be advertised on ECF, because there is now a body of clinical trials that show this is required for some persons in some circumstances. One vendor is about to get a pharmaceutical license for this and has produced evidence that it is safe, for use as directed. In other words if you need to go higher, then that's OK. Probably best not to start right at the top end, though.
As stated, registered vendors can do what they like on their sites apart from selling pure nic.
Note that there is a demonstrated difference in nicotine tolerance between individuals of a factor of 10 (shown first here on ECF), and it has no relationship at all to what a person smoked previously. In other words a few find 6mg the most they can handle, and in contrast a very small number of people cannot succeed with less than 60mg strength (and get no symptoms of nic OD at all). This is absolutely not recommended for anyone except experienced vapers. Actually it would probably be better to trial WTA liquid before going to such a strength.
There is a typical bell curve graph of nic strengths that people need, especially at the start: most are around 18mg. It doesn't matter if you need to go higher, if there are no indications of over-consumption - it just means you need more. It probably isn't a bad idea to reduce a bit over time; many do.
As the OP says, mixing low + high will get you somewhere in the middle, as required. Also you can use a lower strength at night, before bed, that way. Some need to go low in order to sleep better.