Pivot Table data calculation

Is pivot table data controllable beyond sum, count, etc? Simplified, I have a table in EXCEL (2002)with column headings listing machine, good pieces, bad pieces, scrap %, and a dozen other data. There are multiple rows per machine, added daily. I have a pivot table that will summarize good and bad pieces per machine. I want to show an overall scrap % for the machine. I can only achieve a sum of the scrap rates. E.g., if machine A produced 1% scrap for 10 days, rather than a 1% overall scraprate, I get 10% – 1 from each of the 10 records, summed. Can I get that column intended to show the overall scrap to do so, and not show a total of the individual scrap %s?