Calculate difference between value and total value

Hi, Matt78 

thanks for lbendlin's concern about this issue.

Since you didn't provide a dataset for testing, I assumed some data for testing and hope it helps: 

You can then create a Measure: 

SelectedCustomerGroup = 
CALCULATE (
    VALUES ( 'Sales'[CustomerGroup] ),
    FILTER ( 'Sales', 'Sales'[Customer] = SELECTEDVALUE ( 'Sales'[Customer] ) )
)

Then add a Slicer visual and a Table visual. Here is the test result: 

If the above one can't help you get the desired result, please provide some sample data in your tables (exclude sensitive data) with Text format and your expected result with backend logic and special examples. It is better if you can share a simplified pbix file. Thank you. 

I hope my suggestions give you good ideas, if you have any more questions, please clarify in a follow-up reply.
Best Regards,
Fen Ling,
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