For starters, I wouldn't suggest having a field that is just calculated - you can just run a query that will automatically. With a query such as:
SELECT Field1, Field2, [Field1]-[Field2] AS Difference
If you want it displayed on the form, you can just have an unbound textbox with the control source being something along the lines of:
Storing the difference of the two would just take up space.
HOWEVER, having said that
If, for some strange reason you REALLY REALLY
need to have it stored, you could put some code into the form's BeforeUpdate event to calcutate the difference, or on some other event, such as the OnEnter event of the field.