Use ISNULL() or ZN() to find the blank or null values.
Create a calculated field as -
IF ISNULL([Completed Date]) THEN DATEDIFF('day',[Estimated Date],TODAY())
ELSE DATEDIFF('day',[Completed Date],[Estimated Date])
The above code would fetch difference of days wrt completed date or current date(if null/blank).
Hope it helps you.