Matrix Report Calculation

I have a matrix report and at the end i have two calcs.  Avg of 12 months and Sum of 12 months - no big deal... - it's a

so Rolling Month 12, 11, 10, 9 ~1 How can I calculate the most recent month  / Average Sales using a matrix report....

If YRMonth = MaxYearMonth = Then Average/Max(YearMonth)

Thanks...

SSRS 2008 R2

KDW