Dynamically Update DAX with Slicer

ZW , I think this correct as measure.

Count n/a Percentage =
DIVIDE (
COUNTX ( FILTER ( 'Table', 'Table'[Birthday] = "n/a" ), [Birthday] )
+ COUNTX ( FILTER ( 'Table', 'Table'[Nationality] = "n/a" ), [Nationality] )
+ COUNTX ( FILTER ( 'Table', 'Table'[Gender] = "n/a" ), [Gender] )
+ COUNTX ( FILTER ( 'Table', 'Table'[Age] = "n/a" ), [Age] )
+ COUNTX ( FILTER ( 'Table', 'Table'[Occupation] = "n/a" ), [Occupation] )
+ COUNTX ( FILTER ( 'Table', 'Table'[Address] = "n/a" ), [Address] ),
COUNTROWS ( 'Table' ) * 7
)

Not as column , as first row do not have any N/a and it 71 %

you can have column like

Count n/a Percentage =
DIVIDE (
if('Table'[Birthday] = "n/a" ,1,0 )
+if('Table'[Nationality] = "n/a" ,1,0 )
+if('Table'[Gender] = "n/a" ,1,0 )
+if('Table'[Age] = "n/a" ,1,0 )
+if('Table'[Occupation] = "n/a" ,1,0 )
+if('Table'[Address] = "n/a" ,1,0 ) ,
7
)

But my advice is use first one as a measure