Free Water Deficit

Hi Colleagues

May I ask a question, What does free water deficit means for Hypernatremia? I know the definition but what does it actually means when treating hypernatermia with crystolloids solution. For example if serum Na is 190, free water content of 0.9% NaCl is 19%, 0.45% Saline is 59% and 0.2% Saline is 81%. How this will help in giving free water to a baby weight 4Kg with Na 190 mmol/L, who require approximately 576ml of Free water in 4 days to drop Na slowly to 142 mmol/L.

Many Thanks in advance.