Blanket Sales Order and Cancellation of SO Order

Hi !

We use the BL type of Sales Orders to follow quantity Sales and invoiced.

I only use non-inventory item with SO Invoice without Shipment.

But How could I track the real quantity into customer Order when a credit memo is created for correcting a invoice ?

I E : If we create a child SO Order then invoice it and after that create a credit memo(from AR30100 because of no shippment restriction) for some reason with reducing the quantity invoiced. 

How could the blanket order reflect the real quantity into sales order and invoices linked ?

I could copy my SO into a RC one but it would be linked with my BL Sales order.

Thank for your help