Typically, 48mg and above is considered hazardous material. A lot of vendors have stopped carrying anything over 36mg.
Some labs still provide the high concentration stuff. You need to wear at least gloves for 48mg and good ventilation
might be a good idea. ( some are sensitive to nic juice, just mixing down 48 mg was enough to give me an overdose )
Anything over 48mg, and gloves with eye protection and a fume hood are pretty much mandatory. Some people will
say "I gargle 100mg with no problem"

, but why take the risk.
An easy base to work with is 36mg. Most common ratios are 6, 12, 18, and 24mg. This is easy to cut from 36mg.
Figure it as parts per whole. If you want 18mg juice, divide 36 by 18 that gives you 2. So 1 part nic and 1 part PG/VG
gives you 18mg juice. ( 10ml of 36mg nic and 10ml of PG/VG gives you 20ml of 18mg juice )
24mg is a little trickier, you can't divide 36 by 24 but you can divide by 12 ( = 3 ). So, 2 parts nic and 1 part PG/VG
gets you 12mg juice. ( 20ml of 36mg nic and 10ml of PG/VG gets you 30ml of 12mg juice )
Of course, there are plenty of calculators out there, there is even one for the ipod. Just search ejuice.
You enter your starting nic level and how much you want to make at what level nic you want and it will tell you what
ratios to mix.
Nicotine comes suspended in either PG or VG. You have to decide what ratio PG/VG you want to decide which
would be easier to mix. For example, I like a 70/30 ratio of PG to VG at 12mg. I use 36mg VG and mix 1 part nic to
2 parts PG. That gets me very close without having to try to make real accurate measurements.
Just remember, the flavoring is part of ratio as well. 15% is the most common level of flavoring, some will vary widely,
but 15% is a good rule of thumb. When making your nic base, short one of your components to allow for adding your
percentage of flavoring.
For example: My Cinnabun
15% cinnabun
5% vanilla
5% caramel
75% base
10ml 36mg VG and 12ml of PG leaves me room to add my 25% of flavoring to get me a 30ml batch. ( give or take a little )
It's just like cooking, you don't have to get it exactly right.
You will have to do some experimenting with the flavors you like. Some flavors can change a lot, for good or bad, with just a 1%
change in ratios.