Would you be able to post a packaged workbook (twbx file) or screenshot of the results your getting from the calculation and explain what you would expect to get?
I was not placing COUNTD in the correct place. I was doing something like IF xyz THEN COUNTD(123).
The way that ended up working for me is shown below.
COUNTD(IF(ISNULL([Approving Manager])) THEN [Unique event ref] END)