WHERE inside a calculated measure? (MDX/OLAP)?

Question

SQL Server

I create a cube in SQL Server Analysis Services; the following problem arose.
There is a dimension D and a measure M, connected as many-to-many (which is important).
I need to define a calculated measure using the formula M/Σ(M), where Σ is the sum of all the measure facts associated with at least one member of the current dimension. For example, I have a total for measure M of about 3,000,000, while the following query returns about 120,000:

SELECT [Measures].[M] ON 0
FROM [MyCube]
WHERE [D].[All].CHILDREN

As you can see from the example, I learned how to calculate the correct sum separately - all you need to do is take not the whole cube, but its slice, limited by all members of the dimension.
But here's how to calculate this amount inside a calculated measure? Here is a simplified query example that I thought would work:

CREATE MEMBER [Measures].[Calc] AS
  [Measures].[M] / ( DRILLUPLEVEL( AXIS( 1 ).ITEM( 0 ).HIERARCHY.MEMBERS ).ITEM( 0 ), [Measures].[M] )
  WHERE DRILLUPLEVEL( AXIS( 1 ).ITEM( 0 ).HIERARCHY.MEMBERS ).ITEM( 0 ).CHILDREN

But of course MDX doesn't allow WHERE in a MEMBER declaration.
Can you please tell me if there are other ways to limit a slice for a calculated measure?

Answer 1

As a result, I came to the following universal solution:

CREATE HIDDEN [Total M] =
  AGGREGATE(
    DESCENDANTS(
      AXIS( 1 ).ITEM( 0 ).DIMENSION.LEVELS( 0 ).ITEM( 0 ),
      AXIS( 1 ).ITEM( 0 ).DIMENSION.LEVELS.COUNT
    ) - AXIS( 1 ).ITEM( 0 ).DIMENSION.LEVELS( 0 ).ITEM( 0 ),
    [Measures].[M]
  );

In short, we subtract the root member from the set of members of our dimension, and aggregate our measure over the remaining set.