Calculated Measure - Count number of orders

Hi

I am trying to create a measure to count the number of Sales Orders.

The Order dimension contains over 1 million members.

Order Hierarchy is:

     Order Company

          - Order Number        (I want to count the members in this level)

                - Order Line

I have tride a few different approaches, all of which have been very (VERY) slow. Here is an example

Create Member CurrentCube.[Measures].[Order Count]
 as SUM(
    NONEMPTY(
        DESCENDANTS( [Order].[Order].Currentmember , [Order].[Order].[Order No])
                    , [Measures].[Sales Quantity]
                    )
    , IIF([Measures].[Sales Quantity],1,NULL)
             ),
Format_String = "#,0",
NON_EMPTY_BEHAVIOR = { [Measures].[Sales Quantity] };  
Is it possible speed this up?
Jamie