Flowfield problem

My problem is that I want to calculate a sum of amount in a Master Table based on a Detail table. The amount to summarize is not in the Detail table but in a Third Table which has a TableRelation with Detail one. How can I design the FlowField in master table which calculates an amount of a supplementary table based on the data of Detail Table. The problem is that I want to involve more than one tables in the calculation formula of Flowfield. Thanks in advance.

You cannot involve more than one table in your flowfield. Did you think about creating your flowfield calculation based directly on the table where there is your data (your supplementary table?) Sometimes, you have to add one or two fields to do so (ex: repeat the Shipping Agent on an Invoice Line, even if it’s in the header).

You can not do it coz a flowfield can not be included in sumindex field of the detail table.