In FDI how to execute detail level calculations before aggregation of data. Using a custom subject area I am trying to execute detail level calculations on data before the data can be summed up.

We have detailed shipment records - one line per item shipped - with a status of either 'shipped' or 'returned' and positive values for item price, shipping charges, and tax. Depending on the status and invoice date or return date we want these values to be either treated as positive ('shipped') or negative ('returned') values. 

Currently, FDI is accumulating the values before executing the the calculations and is summing up all numbers first, then determining whether the values are negative or positive.  Which is not correct.

We need the values to be net - sales less returns. The detail values need to be set to positive or negative before they are aggregated.

Is there a way in FDI to do the above?  Or would we better off doing the detailed calculations in a dataset and then using the dataset to produce the FDI workbook?

Any help appreciated.

Paul