Calculate or sum a calculated fiel depending on the hierarchy

Hello everyone,

First at all, I use the PowerPivot version contained in Excel 2013.

I have a database containing :
- one row by resource containing the real quantity, the plan quantity and the value
- resources are classified by production model
- each production model contains a reference ressource
- and production models are classified by family

I want to calculate the difference of production, so I calculated the relative difference of production for the reference ressource, and then I multiply this relative difference by the sum of all the values of all the ressources contained in the production model.
This calculation works very well when I display the results in a pivot table with the production model only in the rows.

But the problem appears when I want to calculate this field by family. The difference of production for one family is equal to the sum of all the difference of production for all the production models contained in the family.
But here PowerPivot sum all the rows corresponding to the family and then makes the calculation.
How can I ensure that every difference of production has to be calculated independently ofr each production model ?

I tried some things with the function FILTER(), hoping that it will make an iteration but it doesn't work.

Thank you in advance for your help
You will find a excel file attached illustrating my problem, I tried to be as clear as possible, tell me if it's not

Aeonn