newbie: calculation related mainform and subform only on current month

[cuongvt]

I have a stock control database. I created a query to find out total remained quantity of each product on only current month by using the delivered quantity on current month to subtract the quantity took out from the stock on current month. It works fine as I want.

But now I want one more thing:
I've just created a field "remained" in "products" table and a mainform named "products" with two subform: "stockout subform" and "subdelivery". In the main form have a "remained" control and this control have a control source "remained" in "products" table.
What I want is at "products" form, when I input the delivered quantity and the quantity that took out from stock, the "remained" control on the main form will display the remained quantity on current month. Of course, this control will write down the remained quantity to "remained" filed of "products" table.
I posted my file for reference.
thanks in advanced.

 

[jv2]

To me it sounds unnecessary to have the "remaining" field at all... why do you need this in the table? Everytime you need the figure, you can just calculate it... either in a controlbox in your form... or in straight the query for your reports... or not???