Help with Excel percentages

Hello,

Please bear with me as I a very new to this type of syntax with that said here we go.


I have four date columns in Excel like so:

PROJECTED_CHECK_OUT_DATE   ACTUAL_CHECK_OUT_DATE   CHECK_IN_DATE    PERCENTAGE_STAY
         3/20/2010          3/10/2010                 3/5/2010                 50% 

CHECK_IN_DATE date into hospital, due to medical condition calculate PROJECTED_CHECK_OUT_DATE,
leaves hospital ACTUAL_CHECK_OUT_DATE. Need formula to figure hospitalization stay was only fifty percent of the PROJECTED CHECK OUT DATE if his ACTUAL_CHECK_OUT_DATE is more or less PERCENTAGE_STAY
percentage will change accordingly. ALL HELP IS MOST WELCOMED. Thank you.