How to override SubTotal for Calculated Measure with Scope statement - Please this is urgent.

Hello experts.

I know this should not be tough and I knew how to do this but apparently I'm out of touch. I need somebody to get me out of my hole.

I need to calculate the subtotal for ALL (visible) MaterialIDs in the current month, that belong to the current Plant ID. The measure is a calculated measure called TestValue.

Here is my SCOPE statement:

SCOPE  ( [DimMaterialMaster].[Material ID].[ALL], [Measures].[TestValue] );  
    THIS = 
        SUM
        (
            (
                [DimPlantMaster].[Plant ID].CURRENTMEMBER, 
                [DimMaterialMaster].[Material ID].MEMBERS -- (also tried Children and siblings
            )   
           ,[Measures].[TestValue]
        );  
   BACK_COLOR (THIS) = 5884570;                                                     
    FORE_COLOR (THIS) = 0;    

END SCOPE;  

I'd really appreciate a quick hint.

Thx,

Dirk