Buy the highest shots, 20 mg, best buy for your money.
Then you calculate: if you want 500 ml juice with, let's say, a Nic-strength of 3 mg/ml you have to calculate: 500x3=1500 mg nicotine.
You have 20 mg/ml in your shots. so you need 1500/20=75 ml from your shots.
Each shot has 10 ml. So you need 75/10=7,5 shots.
Rob (from TPD-Germany...)