I'm new too so I did it the easy way. I ordered (from WL) 1 250ml of 18mg nicotine in PG and 1 250ml of 18mg nicotine in VG. I then mixed them together in a 4 cup measuring cup (and stirred with a glass candy thermometer) and poured them back in the bottles they came in. I did mix back and forth between the bottles to get the best total mix.
So now I have 18mg nicotine at 50/50. Adding my flavoring will change the ratio to somewhere around 60 pg/ 40 vg or maybe a little higher on the pg depending on how much flavoring I add. It will also lower the nicotine to around 14.5mg (at a 20% flavoring mix).
I'm really not that concerned about the exact pg/vg ratio or the exact nicotine level at this point (although I may be later). I've made two small batches of ry4, changing the second one a little, and they both turned out very good. Of course I got my starting recipe on this site, which helped. A lot!
Also, be sure to get some graduated cylinders and syringes. I should have bought a few more the first time around. Oh well, it's a learning process, and lot's of fun too!