Many slip and summary fields contain calculated values which can be overridden by positioning the cursor at the field in question and pressing the F4 key or double clicking any field indicated by 
. The background colour of the field will change to indicate the calculated value has been overridden.
To revert to the calculated value press F4 or double click in the field. To revert all fields on the form to their calculated values, click on the Recalculate button. In either case you will be asked to confirm the return to the calculated value.