Nav Sales Order Profit in a Pivot Table Problem

By linking the sales lines and sales header I can get the sales lines by customer into a Pivot Table.
As NAV has sales line value but not sales line cost there needs to be a calculation column in the pivot table for 'quantity x unit cost'.
Then another two calculated columns to work out the profit and profit %.
That all works fine until the sales lines are collapsed to show just the sales header values for using in analysis with the Slicer.
As the Calculated columns can't sub-total per order but calculate along the line they give the wrong totals.
(Sum of sales order qty x sum of sales unit cost = Total Order Cost) which is wrong.

Is there a better way of getting the sales order values that sit on the sales statistics screen out of NAV?
Or Is there a better way of calculating the line cost either before it gets to the pivot table or in the pivot table?

Lastly why O' why can't NAV just have the sales order values as Flow Fields from the Sales Header?
Once again I bang my head on the table in frustration.

Regards Chris