Crystal Reports Average of a running total

Message

I have a report that is grouped by CompletionID.  Each completion ID has a production record for each date.  I need to average the most recent 7 days of production greater than 0 for each completion.  I tried using a running total to limit the number of records for each completion ID to the 7 most recent, but I can't figure out how to average the production for only the most recent 7 dates. 

Completion ID 2246                                    01/26/2013                                                                                             
14.81
01/25/2013                                                                                               
8.73

01/24/2013                                                                                             
13.29

01/23/2013                                                                                             
11.78
01/22/2013                                                                                             
13.98

01/21/2013                                                                                             12.44
01/20/2013                                                                                             
40.64

Completion Avg                                                                                       5.85

The completion average for the 7 dates selected should be 16.52, not 5.85.

Thanks in advance for your help!