Fulfillment form and custom calculations

We have implemented a duration capability in sales orders (can order a subscription of x months for example). To do this, I added a field that manages item ordered quantity. We use the original 'quantity' field to manage overall unit quantity so we can process duration-based items. For example, if a user orders 3 12 months subscriptions (where the unit is a single month), then the custom 'item quantity' field would hold '3' and the original 'quantity' field would hold '36' (3 items X 12 months each).

My question has to do with partial fulfillments. Under a normal condition, if you partially fulfill an item, you would change the 'quantity' to reflect the number you actually want to fulfill. When you return to complete the remaining fulfillment, NetSuite displays the remaining value in the 'quantity' field.