Calculate difference between forecast (constant) and monthly values

Hi Anonymous 

You can try this measure,

diff = 
var _forecast= MAXX(FILTER(forecast,forecast[Location number]=MAX('Table'[Location number])),[Forecast value])
return _forecast-MAX('Table'[Value])

if you need calculated column, try this,

Column = 
var _forecast= MAXX(FILTER(forecast,forecast[Location number]=EARLIER('Table'[Location number])),[Forecast value])
return _forecast-'Table'[Value]

Best Regards,

Community Support Team _Tang

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