Hello Claudio, 

The FlatDataSource does not currently support calculated measures. Calculated measures are only supported on XMLA data sources.  

If you need a calculated measure then I recommend adding another field to your underlying data source that represents the calculated measure.  For example:

public class Sale
{
 public double AmountOfSale { get; set; }
 
 public double DoubledAmountOfSale { get { return AmountOfSale * 2.0; } }
}

We have a online topic that walks through on creating calculated measures. You may also refer to the following sample for more details. See below. 

PercentageAggregator.zip

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Let me know if you have any questions. 

https://es.infragistics.com/help/wpf/xampivotgrid-us-customaggregators

Aggregator example with weighted sum help class. (although it has some issues that were fixed in a comment below)

https://es.infragistics.com/community/forums/f/ultimate-ui-for-wpf/45131/calculated-custom-measures-for-flatdatasource-in-xampivotgrid/331675#331675

The only thing that was missing from those two references is how the aggregator should be actually applied to the FlatDataSource, because in the first example, the agregator is applied to CubeSettings[0] but that list was empty thus throwing a null reff exception.

For me it looked something like this.

CubeMetadata cube = new CubeMetadata();
//weirdType is a dynamically generated type, use typeof(**yourtype**).FullName for other cases.
cube.DataTypeFullName = weirdType.FullName;
cube.DisplayName = "weird type";
DimensionMetadata testMetadata = new DimensionMetadata
{
     SourcePropertyName = "prop1",
     DisplayName = "test",
     DimensionType = DimensionType.Measure,
     Aggregator = new TypeBuilderNamespace.IgWeightedAverageAggregator("prop1", "weightProp")
};
cube.DimensionSettings.Add(testMetadata);
FlatDataSource dataSource = new FlatDataSource();
dataSource.CubesSettings.Add(cube);
//data is a collection of weirdType objects
dataSource.ItemsSource = data;
//then set that dataSource.

so the weird type has a property "prop1" that has values that should be weighted by "weightProp". the "test" displayed doesn't exists as a property of weirdType