Count of unique entries in very large data set

I have a very large data set, ~39,000 rows of 18 columns.
the data set is probably going to grow larger.
I have a Pivot table of the set.
I want to count the unique values of the combination of two of the columns and have the equation:

Formula:

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In row two of the last column.
I then fill down, but excel hanges because the calculation takes so long.
I am very familiar with VB and could do an Add-in if that could work.
Any other ideas on how to get the nuique count from this data set?

Thanks.