VBA code to Fill Down in a filtered list only to visible cells.

I am trying to create a macro that will fill down in a column based on a
filtered column in a list. I'm creating a formula in a separate column which
I want to fill down to only the visible cells based on the filtered field.

i.e. I am filtering on Column C to show only values 'X'. Then in Column D, I
want a formula I create in the first record to be duplicated to all of the
visible cells/rows in that column to the bottom of the list.

Any ideas?

Thanks in advance!
Training Goddess