Milestone billing with Partial Dispatch

Scenario..

1) The material we have is a large machine tool

2) We create a sales order for stage wise payments and raise down-payment requests for the advance and various stages of manufacture.

3) The M/c is assembled and checked for performance...

4) Since the machine is of large size it is now dismantled and dispatches of parts of the m/c take place.

5) We are not sure at the time of sales order as to what would be final configuration of the knocked down machine

6) The machine is packed -- child material of various sub-assembles are clubbed together and a packing list is prepared..

7) Delivery of these packs are made thru PS

8) Now I want to bill this m/c with 70% of the payment to be realized at the time of dispatch..

However the 70% of the payment to be realized has to billed in 4-5 lots as per the delivery of the individual packs delivered.

How do I Do the final billing ??

Kindly Help

Regards

Somnath