Above, Donald Miller writes: "If you are at four minutes then a one and one half stop adjustment would be ten minutes." This is just wrong.
If you are at four minutes, then one and one half stops would be 12 minutes. Donald made a rookie mistake. Each stop is a doubling of the light from the stop before it. Hence, when you go from 4 min to the next stop it gives you an exposure of 8 minutes. The 8 minutes is then doubled to give a two stop increase to 16 minutes. It is not 4+4+4, it is (4x2)2. Half a stop between 8 and 16 is 12, not 10.