VBA code for data reduction of csv files

Hi all,
Apologies for what is probably a very basic question, but one I have been struggling with for days. Maybe someone can help?

I have several thousand huge csv files that I need to reduce by data averaging. I want to open each one in excel, and then I want to average every 12 datapoints and put the output in the next column. I have no problems with any of the other parts of the macro, but I can't work out how to do the averaging - I have not had much success with offset, this is what I have tried:

Dim strFormulas As Variant
strFormulas = "=SUM(OFFSET(R7C1,(ROW()-ROW(R18C1))*12,0,12,1))"
Range("C19").Formula = strFormulas
Range("C19:C12000").FillDown

Is there a better way, or can anyone help?

Thanks so much