Percentage Difference by selecting reference from slicer

Hi arlequin71 

You need to add an independent Product table to the model. This means there is no relationship between Product table and Table1. Use Product table's product name column in the slicer. 

Then modify the measure into below

Avg Price % difference from selected product = 
VAR __SELECTED_PRODUCT = VALUES('Product'[Product Name])
VAR __BASELINE_VALUE = CALCULATE([Avg Price], 'Table1'[Product Name] IN __SELECTED_PRODUCT)
VAR __MEASURE_VALUE = [Avg Price]
RETURN
    IF(
        NOT ISBLANK(__MEASURE_VALUE),
        DIVIDE(__MEASURE_VALUE - __BASELINE_VALUE, __BASELINE_VALUE)
    )

Best Regards,
Community Support Team _ Jing
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