DAX moving average using a measure

Question

I am trying to create a 12 month moving average for headcount data using the following DAX (following a blog post by Javier)

http://javierguillen.wordpress.com/2011/09/13/calculating-moving-averages-in-powerpivot-dax/

12 Month AVG Headcount:=
IF(COUNTROWS(VALUES('Date'[Month Name]))=1,
CALCULATE(AVERAGE([Ending Headcount]),
DATESBETWEEN('Date'[Date],
FIRSTDATE(PARALLELPERIOD('Date'[Date],-11,MONTH)),
LASTDATE(PARALLELPERIOD('Date'[Date],0,MONTH)
)
),
ALL(DATE)
)
)

The problem is that the Ending Headcount measure is a calculated measure and not a column value. Therefore I keep getting the following error message  1    "Measure 'Monthly Headcount'[12 Month AVG Headcount] : The AVERAGE function only accepts a column reference as the argument number 1"

I don't really want to put the logic for Ending Headcount into the query above. Is there any way around this?

btw...I did try and do a SUM(Ending Headcount) / CountRows(......) and SUM only accepts a column reference as well....

Thanks for your help...Frank

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Frank