Crystal Reports Identify customers best month

Message

     Good afternoon,

I need to build a decline account report.

What would be the best way of identifying a customers BEST month based on their quantity purchased? (so simply need to identify which month, in the last 2 years, had the highest quantity of units)

I want to go back as far as 2 years.

my table contains fields YEAR & PERIOD & QTY

the YEAR data is:  31 = 2011,  30 = 2010,  29 = 2009, and so on.

the PERIOD data is: 1 = Jan, 2 = Feb, etc...

the QTY data is at line level, so I will need to SUM this by customer.

Any ideas?

I currently have the report set up as:

Header     CUSTOMER     CURRENT MONTH    LAST MONTH     BEST MONTH
group1     CUSTOMER A          29 units               84 units                ?
                CUSTOMER B         117 units            209 units                ?
                CUSTOMER C     
Etc, etc

Can anyone advise on a formula for me?

Thanks in Advance

Mike