You have to understand that there is a series of events that occur when the user tabs out of a field. The entered data is formatted, validated, and then "Committed" (applied) to the field. It is at this point that calculations in the other fields are triggered.
The new field value does not exist before it is committed. If you were to trigger the calculation in the "Sum" field early, like in the change event, it would not work properly because the correct values are not yet there.
If the user hits the "Enter" key instead of "Tab", this same progression happens, only the focus stays on the field. I think that this is what you are looking for.
Thom Parker
WindJack Solutions
www.windjack.com