When using xamPivotGrid with a FlatData source, is it possible to customize the aggregation function to be used?

For example we need an employee efficiency data to be shown in  xamPivotGrid, with three Measures: 

'Productive Hours' (int value), 'Total Hours' (int value) and '% Efficiency' (decimal %).

where '% Efficiency' = 'Productive Hours' * 100 / 'Total Hours'.

By default the '% Efficiency' measure is also just adding up all the individual percentages, rather than applying the same formula on the aggregates. And so after doing much research, I ran into this post and I followed the instruction to succesfully achieve the above functionality using Custom Aggregator. However this may not be a viable solution for my application since the Dynamic Measures would be based on formulas coming in from the database via WCF service. Hence I can't really create a static Custom Aggrgegator.

Also I see that you can specify AggregatorType = Calculated when defining DimensionMetadata. Can you please its usage and if there is a way to specify formula based on other Measures.

Thanks.