Having tried mostly PG based and mostly VG based, I really like 50/50. As far as the nic level, you can get it high, such as 24 mg, and bring it down with no-nic PG and VG, equal parts in the case of 50/50.
For example, if you wanted to make 18 mg/ml out of 24 mg/ml, 18/24 = 0.75 or 75% so that's how much of your final juice would be 24 mg nic juice. The rest would be equal parts of no-nic PG and VG for 50/50.
Hope this helps. I get all my nicotine base from myfreedomsmokes.com where they publish the lab results of their nic base. For short term use, I get liters of 24 mg and for long term storage 100 mg/ml. Be extremely careful working with 100--in a WELL ventilated area, and with eye protection and nitrile gloves. MFS also carries concentrated flavors from FlavorArts and TheFlavorApprentice/ThePerfumersApprentice.