helper column to be used in pivot table

[craigexcel]

I have a large dataset (for me anyway) that I need to create a Pivot Table for. The data table contains journal entry data for 2014Q2. Since I need to count unique journal entry numbers, between the months (since some journal entry numbers repeat across months), I have added a helper column. I seem to be running into a processing speed(?) issue when attempting to get the helper formula copied to all nearly 70,000 rows. (My laptop has 4GB RAM.)

Is there some way to reduce the overhead with the formula I'm using so it's not such a processing 'load'? Any other thoughts as to how better to handle this?

Current formula I'm using is: =COUNTIFS($F$7:F7,F7,$K$7:K7,K7)
where:
column F - contains the journal entry numbers, and
column K - contains the period (Apr - Jun)

The idea is, of course, to populate the helper column with a 1 for the first journal entry (only), for a given month.

I'm using MS Office Pro 2010.

As always, thanks for your insight!
Craig