use a formula with a shared variable.  Every time it an unsupressed record comes along, then increment, this will give a count.  But since you are looking for a distinct count, what I would do is uppercase the value that your distinctly counting, and place it in a delimited string, this way you can search the string to see if that value has already been 'seen'.  Then when it comes time to display the count, split the string on the delimiter, and check the upper bound.

really simple.