Profit Margin by Sales Rep Summary

I am trying to track my sales-reps' individual over all profit margin and running into some trouble.

I first tried to do this by creating a saved search that averages the transaction-level profit margin per transaction over a specified date range. (Last month, This month, etc).

This seemed to work fine until a transaction popped up that lost money. There was a transaction where a customer had to basically give a part away for free to make up for an error he made in the previous order. So we sold a part (that cost us money) for zero dollars. This caused the transaction-level profit margin for that transaction to be a HUGE negative number. Then when the report took the average of all of the transaction-level profit margins, it was largely thrown off by this huge negative percentage number from that one transaction.