SSAS Calculated Member for measures that have the same Dimension

Question

I am trying to create a calculated measure that subtracts measures that are in different groups, but only if they have the same dimension member whatever that may be. (edit - essentially exclude UnknownMember numbers in the subtraction)

I have tried using Scope:

CREATE MEMBER CURRENTCUBE.[Measures].[CalcField] as NULL;

SCOPE([Dimension1].[DimensionField1].MEMBERS);
    [Measures].[CalcField] = [Measures].[a] - [Measures].[b];
END SCOPE;

I have also tried using a Tuple without success:

CREATE MEMBER CURRENTCUBE.[Measures].[CalcField]
AS ([Measures].[a] - [Measures].[b], [Dimension1].[DimensionField1]);

But I think there is something fundamental that I am missing

Edit

col ab dimension
1. 9 0 x
2. 0 2 x
3. 1 5 null

If you aggregate those rows I want the answer 7 not 5

Using a simple CASE WHEN works if the dimensions in use, but otherwise it blindly subtracts everything again

CASE 
  WHEN [Dimension1].[DimensionField1] IS [Dimension1].[DimensionField1].UnknownMember THEN 0 
  ELSE [Measures].[a] - [Measures].[b]
END

Using Aggregate works at the high level, but then when I use the Dimension I get no per member results

Aggregate(
  EXCEPT(
    [Dimension1].[DimensionField1].Members, {[Dimension1].[DimensionField1].UnknownMember, [Dimension1].[DimensionField1].[All]}
  ),[Measures].[a]) 
- 
Aggregate(
  EXCEPT(
    [Dimension1].[DimensionField1].Members, {[Dimension1].[DimensionField1].UnknownMember, [Dimension1].[DimensionField1].[All]}
  ),[Measures].[b]) 

Workaround Solution
I am sorry if I was not clear on my problem, but I have ended up solving this problem by putting the data I need in at the ETL stage rather than calculating it in the Cube

Answer 1

With Member [Measures].[CalcField] as   [Measures].[a] - [Measures].[b]

Select [Measures].[CalcField]
on columns,
[Dimension1].[DimensionField1].MEMBERS    
on rows 

from [cube]

Answer 2

Did you try either Summing or Aggregating your second snippet?

CREATE MEMBER CURRENTCUBE.[Measures].[CalcField]
AS 
 Sum(
    [Measures].[a] - [Measures].[b], 
    [Dimension1].[DimensionField1]
 );

CREATE MEMBER CURRENTCUBE.[Measures].[CalcField]
AS 
 Aggregate(
    [Measures].[a] - [Measures].[b], 
    [Dimension1].[DimensionField1]
 );