Dynamic Table Top10 Grouping & Other

Rocky2121 

I think this can be solved by treating it more like a dynamic ABC classification.

We need a table to hold the customer segments.  You can create it with this dax code.  I did top 10 just so the screen shots would fit better.

Customer Segments = 
DATATABLE (
    "Segment", STRING,
    "Order", INTEGER,
    "Rank Start", INTEGER,
    "Rank End", INTEGER,
    {
        { "Top 30", 1, 1, 10 },
        { "Other", 2, 11, 9999999 }
    }
)

Then we need a ranking measure to set the rank for the cusomters.  This will be used to put them in the correct segment.  I am basing mine of [Sales].

Customer Rank = 
RANKX ( ALLSELECTED ( Customer ), [Sales] )

And a measure to calculate the [Sales] for the customers in the Segments.

Customer Segment Sales = 
VAR _CustomersInSegment = 
    FILTER (
        ALLSELECTED ( Customer ),
        VAR _RankOfCustomer = [Customer Rank]
        VAR _SegmentForCustomer =
            FILTER (
                'Customer Segments',
                NOT ISBLANK ( _RankOfCustomer )
                    && 'Customer Segments'[Rank Start] <= _RankOfCustomer
                    && 'Customer Segments'[Rank End] >= _RankOfCustomer
            )
        VAR _IsCustomerInSegment =
            NOT ISEMPTY ( _SegmentForCustomer )
        RETURN
            _IsCustomerInSegment
    )
VAR _Result =
    CALCULATE ( [Sales], KEEPFILTERS ( _CustomersInSegment ) )
RETURN
    _Result

Put the Segment from the Customer Segments table and the customer name in a matrix along with the [Customer Segment Sales] measure:

And this will adjust as you make selections:

I have attached my sample file for you to look at.

This is based on this pattern from SQLBI.
https://www.daxpatterns.com/abc-classification/