Right'o

I have a Form where orders are placed, and as many sub forms as there are items.

My subform's are as follows.

ID
Name
Type
Stock(Hidden)
Used(Hidden)
Available

I hope to calculate stock by doing the following.

Available = [Stock]-[Used]

My problem is. The "Add order and Exit" button wont change the box.
Neither will AfterUpdate from the controlling combo box.

Is there a way to automaticly update it?
Or if possible tell me where im going wrong.

Thanks before hand.