user-defined fields in cost calculation

Hi, I'm working with process order in customer service process.

I would like to make a cost precalculation based on a user-defined field.

To let you know.

I created a new user-defined field.

I affected to it a new parameter ID so that this user-defined field value can be used in formulas.

I created a new formula with my parameter ID.

This is allowed for costs.

Now in my work center I defined these data for costing

price for pre calculation is defined for IN9020 and cost center.

Now in my service order I fill the required field.

I would have thought that according to formula, SAP would use this value (6 PC through the formula) to calculate costs.

This is not the case.

Did anybody already use this?

Regards

Olivier