Header Condition-Item wise

In case of sales order how to divide header condition to item item wise evenly.

I have one header condition which is entered manually at the time of sales order creation. When i m taking same material in 2 line item that header is dividing properly in line item wise, but when i m taking two different materials the header condition is not dividing properly.

Kindly suggest me what is configuration for making header condition to be divided propertionately line item wise.

Thanks in Advance.

Sunita