Power BI - Calculate Percentage % of Total Value

Create a Measure, Use this formula
DataSet Table is Combined_HistoricalSales.
Get the Sum of TotalAmount for the ProductClass. Store it in var bigTotal, then divide by Sum of TotalAmount. Multiply the result by 100.

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MeasureName = var bigTotal = SUMX(ALL(Combined_HistoricalSales[ProductClass]), CALCULATE(SUM(Combined_HistoricalSales[TotalAmount])))
                    return DIVIDE(SUM(Combined_HistoricalSales[TotalAmount]),bigTotal)*100


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To get percentage totals of items within a class
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% of Class Sales = var bigTotal = SUMX(ALL(Combined_HistoricalSales[StockCodeDesc]), CALCULATE(SUM(Combined_HistoricalSales[TotalAmount])))
                    return DIVIDE(SUMX(ALL(Combined_HistoricalSales[ProductClass]), CALCULATE(SUM(Combined_HistoricalSales[TotalAmount]))),bigTotal)*100