100 mg/ml (10%) nicotine is relatively safe to use if you use eye and skin protection. It is also easy to calculate with.
For 12 mg/ml result, use 12% 100 mg/ml in your final solution, i.e. if you're making 10 ml, 12% of that is 1.2 ml. Note that because your nicotine base is 100% PG, you...