Target cost for co-products which can be substitute

Question

Hi,
Is there somebody who is working with co-products which can be different each time you consume raw material?
Here is the situation when we want to analyse variance Actual vs Preliminary cost.
We use Production Order with Co-Products.
When we create a Plan Order, the order costs and qty are calculated for the order header.
Actual costs and qty are charged to the order header as they are incurred.
PLAN: 
Main co-product   900 PC
Co-Product A         50 PC
Co-Product B         50 PC
                           1,000 PC
Consumption:
Raw material         250 PC
ACTUAL:
Main co-product    800 PC
Co-Product A        100 PC
Co-Product B        100 PC
                           1,000 PC
Consumption:
Raw material         260 PC
An apportionment structure is used to apportion the costs and qty to the primary products and co-products.
PLAN
                                                          Raw Material
Main Co-Product   90% * 250 PC = 225   PC  
Co-Product A          5% * 250 PC =   12.5 PC
Co-Product B          5%  * 205 PC =  12.5 PC
                                                            250 PC
ACTUAL
                                                          Raw Material
Main Co-Product   80% * 260 PC =   208   PC  
Co-Product A         10% * 260 PC =    26   PC
Co-Product B         10%  * 260 PC =   26   PC
                                                            260 PC
Variance calculation and settlement are then performed at the level of the order item, not at the header level of the order.
Here is the target qty calculation made by the system.
Main Co-Product  250 PC * 800/900 * 80% = 177.78 PC
Co-Product A        250 PC * 100/ 50 * 10% =    50.00 PC
Co-Product B        250 PC * 100/ 50 * 10% =    50.00 PC
                                                                           277.78 PC of raw material
Since variance calculation is performed at the order item level, that means you can not substitute one Co-Product by another one. 
In our business, we can substitute Co-Product by another one.  In this example we planned to make a total of 1,000 PC finished products and we produced a total of 1,000 PC finished products.  So the target quantity for the consumption of raw material should be 250 PC, not 277.78 PC as calculated by the system.
Any idea?
Regards,

Answer 1

Hello Ajay,
I think  i am missing some thing here. For Configurable product or VC materials there is no standard cost. Costing tab 2 standard cost is zero.
When a make to order sales order is created with this VC materials, based on system settings and quantities system picks up the correct BOM and routings. Through costing funtionality with in sales order , the sales order need to be costed and marked.
This sales order cost is used for  valuation of material movements including 101 from corresponding productuon order and 601 for delivery.
As far as i know for VC material target cost has nothing to do with Standard cost, because there is no standard cost at all for the material.
Please clarifty
thanks
sapfreek