how to calculate two 'Count' columns

I am trying to:
1) subtract the difference between two columns
2) get a percentage difference between the same two columns

Here are my columns on the results tab
Search type = transaction

(1)
Field = Number
summary type = Count

(2)
Field = Fulfilling Transaction : Number
summary type = count

Now what I would like to do is put them into a formula that will subtract them

EX.
Field = Formula (Numeric)
Summary type = count
Formula = count({number}) - count(Fulfillingtransaction.number)

Every time I try this I keep getting an ERROR : Invalid expression.

How would I accomplish this?


--------------------------

ALSO I have found one other issue.
When you count the results in an individual column it does not count duplicates.