How can I setup an Overhead to get absorbed to the completed qty. in shop order?

The cost will be based on the net weight of the product. For example: Per kg overhead is Rs. 8 and if the product net weight is 5 kg then the overhead should be Rs. 40 (Rs. 8 * 5 kg).

When I setup a General OH, it's getting absorbed to the Lot Size irrespective of the completed qty. in shop order.