A user enters the quantity of widgets that they produced.  They hit the save button.  The production data is stored in table separate from the widget table.  Once the user clicks save, I want the BO to update the On Hand total of the widget in the widget table.  I am thinking that I would place some code in the AfterSave() method to make this happen.  Here's the rub...what if the user entered the wrong quantity (not that this ever happens--I am just doing some just-in-case type of analysis here  ?  The user finds their production data entry and clicks edit, then proceeds to change the quantity and clicks save.  Now, what is the best way of handling this in order to update the widget table with the difference in quantity?

I have some ideas from previous non-SF days, but is there a good SF way to do this?

Thanks,
Bill 

Let me see, then...to gather the widget on hand quantity I would run a query (in a view) that would do something like this:

Production table (get total of the widgets made) - Shipping table (get total of the widgets shipped) = Total OnHand Quantity

What about inventory counts when actual and on hand do not match?  How are those handled?  Inventory adjustment table?