Approximate free water deficit- The easiest way to calculate is

1) If the sodium is 10% increased, there is a deficit of 10% TBW(Total body water)

Ex. If sodium is 154 in a 70 kg female, which is 10% more(compared to 140 which is normal), there is a deficit of 3.5 liters(10% of total body water).

This is a quick way to approximate the free water deficit.