How to calculate percentage based on two data sources

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Hi KarolinaZet 

1.You can create a calculated table first.

Country=SUMMARIZE(Source2,[Country Name])

 2.Then create a measure

Measure =
VAR a =
    CALCULATE (
        COUNT ( Source1[BAPIBUS1006_HEAD_BPARTNER] ),
        Source1[BAPIBUS1006_ADDRESS-Country] IN VALUES ( Country[Country Name] )
    )
VAR b =
    CALCULATE (
        COUNT ( Source2[Customer ID] ),
        Source2[Country Name] IN VALUES ( Country[Country Name] )
    )
RETURN
    DIVIDE ( b, a )

Best Regards!

Yolo Zhu

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