If you want the nicotine hit: Generally, 24mg works for me. If it seems a little weak, it's probably not really a full 24mg. You can get down to around 14mg and still get a decent effect if you only vape once or twice a day for a short time.
I have gotten 36 and 48 mg for mixing, and have tried them both straight. While the 36 is just a heavier hit, the 48 will definitely get me nauseated if I have more than about 10-20 puffs of it. Takes about 10 to 20 minutes to wear off. That 48 just slams you with its throat hit. There is no way I'd try anything stronger than 48, I don't see the point. And of course, there is such a thing as nicotine overdose, and it's easy to get it at higher doses, or by leaving significant amounts of eliquid on your skin. First signs are that it makes you flushed, sweaty, strongly nauseated, and anxious. So avoid higher concentrations of nicotine than what feels right for you.
You can dilute with any 0 nic liquid, as you have, or with Propylene Glycol (PG), readily available online, or Vegetable Glycerin (VG). You can find VG bottles in the cake decorating section of most hobby stores.
Why don't you look around in the New Member's forum
New Members Forum - E-Cigarette Forum and ECF library forum
The ECF Library - E-Cigarette Forum
they are very helpful sources of a lot of information gathered in one place, including comprehensive guides and tips and tricks.
In your case, you could also go to the do it yourself forum (DIY)
http://www.e-cigarette-forum.com/forum/diy-e-liquid/, particularly the stickied articles at the top of the forum, and read about mixing and check out the eJuice calculator they have made for measuring out different eLiquid strengths- it explains all about exactly how that works.