Recalculating PPM

I am doing a scorecard, part of which I am scoring PPM (Part Per Million). That formula is: =1,000,000/SUM(#of parts shipments)xSUM(#of those shipped parts that were rejected). MY QUESTION IS: HOW DO I HAVE MY MONTH TO MONTH WORKSHEETS CONTINUALLY RECALCULATE ONGOING THROUGH THE YEAR. Thanks for any help... Suzy