So I have a bunch of rows in a query table that shows orders from a certain timeframe and whether that order had a "defect" or not. I have started a pivot table on that table to summarize the # of orders and # of those orders that were defective by day (trying to start a p-chart).
Is there any way to create a 4th column in the pivot table that calculates the percent defective (Orders with Defects/Count of Orders) from the two existing columns?

Sure,
Based on this site, these are the steps:
Once there give a name to your Calculated Field and set your formula to
Orders with defect / orders(tip: select the fields from the list to make sure you get the names right).One last note: I believe calculated fields perform additions by default.. I noticed your 2nd column is a "count", I believe the calculated field will sum that column (before dividing) instead of counting it.