It's easy math.
Let's say you have 36mg juice. 36/2=18. So 1ml of 36mg juice, and you add 1ml of PG or VG, you've diluted it by half, resulting in 2ml of 18mg juice. 36mg/4=9. So 1ml of 36mg juice, and 3ml of PG or VG, you've diluted it by three quarters, resulting in 4mg of 9mg juice.
The...