TopN Customers Rolling 12 months

This is an interesting problem so I gave it a try.  Let me know if it doesn't work for you as I haven't tested these as I don't really have a good sample data set to work off of.  You'll have to translate the column names I used for your model.

Here is what I came up with for 4 measures assuming you have YearMonth on rows like you have in your result screenshots.

Total Sales = SUM ( FactSales[SalesAmount] )

 Then for total sales last 12 Mo

Total Sales Last 12MO =
VAR _last12MO =
    DATESBETWEEN (
        DimDate[Datekey],
        NEXTDAY ( SAMEPERIODLASTYEAR ( LASTDATE ( DimDate[Datekey] ) ) ),
        LASTDATE ( DimDate[Datekey] )
    )
RETURN
    CALCULATE ( [Total Sales], _last12MO )

then for rolling top 20 sales last 12 months:

Total Sales Last 12 MO Rolling Top20 =
VAR _last12MO =
    DATESBETWEEN (
        DimDate[Datekey],
        NEXTDAY ( SAMEPERIODLASTYEAR ( LASTDATE ( DimDate[Datekey] ) ) ),
        LASTDATE ( DimDate[Datekey] )
    )
VAR _Top20Last12MO =
    CALCULATE (
        SUMX (
            VALUES ( DimDate[YearMonth] ),
            CALCULATE (
                [Total Sales],
                TOPN ( 20, VALUES ( DimCustomer[CustomerKey] ), [Total Sales] )
            )
        ),
        _last12MO
    )
RETURN
    _Top20Last12MO

and finally the percentage is easy:

Percentage of Sales = DIVIDE ( [Total Sales Last 12 MO Rolling Top20], [Total Sales Last 12MO] )

Let me know if this worked.

mattbrice - Hey -  I think this is right. :) I've modified a few things - but you gave me the core idea. THANK YOU!
I'm getting ready to meet with our Sales Manager to verify the numbers but from what I've checked so far it seems accurate.

THANKS SO MUCH! I'll post the final after I confirm. 

Full answer with mattbrice base formulas from a test dataset -  I'm also including the measures for the Count of Customers. I redid the customers count - using the base that matt created. So shout-out to you, mattbrice! :) THANKS!

Count of Customers =
CALCULATE (
    DISTINCTCOUNT ( 'SALES DETAILS'[LinkToCustomerID] ),
    'SALES DETAILS'[SalesTxn Document Type] = "Invoice",
    'Calendar - Transaction Date'[Year] >= 2017
)

Count of Customers Last 12MO =
VAR _last12MO =
    DATESBETWEEN (
        'Calendar - Transaction Date'[Transaction Date],
        NEXTDAY (
            SAMEPERIODLASTYEAR (
                LASTDATE ( 'Calendar - Transaction Date'[Transaction Date] )
            )
        ),
        LASTDATE ( 'Calendar - Transaction Date'[Transaction Date] )
    )
RETURN
    CALCULATE ( [Count of Customers], _last12MO )

Total Outgoing Sales =
CALCULATE (
    SUM ( 'SALES DETAILS'[Sales $$] ),
    'Calendar - Transaction Date'[Year] >= 2017
)

Total Sales Last 12MO =
VAR _last12MO =
    DATESBETWEEN (
        'Calendar - Transaction Date'[Transaction Date],
        NEXTDAY (
            SAMEPERIODLASTYEAR (
                LASTDATE ( 'Calendar - Transaction Date'[Transaction Date] )
            )
        ),
        LASTDATE ( 'Calendar - Transaction Date'[Transaction Date] )
    )
RETURN
    CALCULATE ( [Total Outgoing Sales], _last12MO )

Total Sales Last 12 MO Rolling Top20 =
VAR _last12MO =
    DATESBETWEEN (
        'Calendar - Transaction Date'[Transaction Date],
        NEXTDAY (
            SAMEPERIODLASTYEAR (
                LASTDATE ( 'Calendar - Transaction Date'[Transaction Date] )
            )
        ),
        LASTDATE ( 'Calendar - Transaction Date'[Transaction Date] )
    )
VAR _Top20Last12MO =
    CALCULATE (
        SUMX (
            VALUES ( 'Calendar - Transaction Date'[Month] ),
            CALCULATE (
                [Total Outgoing Sales],
                TOPN (
                    20,
                    VALUES ( 'SALES DETAILS'[LinkToCustomerID] ),
                    [Total Outgoing Sales]
                )
            )
        ),
        _last12MO
    )
RETURN
    _Top20Last12MO

Final Result: