New to MDX. Help Needed with a Calculation Definition in my SSAS 2005 Cube

I'm new to MDX and I don't quite get the hang of it yet, so I'll try to provide all of the relevant information that I can so that you guys can help me.

My company has a lot of "Rolling 12 Month" metrics and I've been able to incorporate all but a few in my SSAS 2005 cube. By "Rolling 12", we refer to the 12-month period that ends with the previous month. So all "Rolling 12" metrics reviewed in December 2005 refer to the period of December 2004 through November 2005.

So for "Rolling 12 Month Invoices", I have the following calculation defined in my cube and it seems to be correct:

SUM( LASTPERIODS ( 12, [Invoice Date].[Invoice Month]), [Measures].[Invoice $])

Now I'm having trouble defining the # of customers in the Rolling 12 month period. I added a DistinctCount measure for # of customers and have been able to use it in the definition of 2 other metrics.

For example, for the # of customers for 2004, I have it defined as:

([Measures].[# of Customers] , [Invoice Date].[Invoice Year].&[2004])

And it appears to fine.

I tried similar variation of the following with no luck:

SUM(SUM( LASTPERIODS ( 12, [Invoice Date].[Invoice Month]), [Measures].[# of Customers]))

Your help is greatly appreciated!

EDIT: I'm using SQL Server 2005 Standard Edition
EDIT: Please let me know what other details you may need about my cube.A Distinct Count measure isn't strictly additive, so try Aggregate() rather than
Sum() to compute it over a set (only works with AS 2005):

>>
Aggregate(LASTPERIODS ( 12, [Invoice Date].[Invoice Month]),
[Measures].[# of Customers])
>>
http://msdn2.microsoft.com/en-us/library/ms145524(en-US,SQL.90).aspx
>>

Aggregate (MDX)

Returns a scalar value calculated by aggregating either measures or an optionally specified numeric expression over the tuples of a specified set.
...

Distinct Count

An aggregation across the fact data contributing to the subcube when the slicer axis includes a set.

Calculations on the set generate an error. Calculations below granularity of the set are ignored.

...
>>

|||Thank you very much!!!