The only way to do this is by using a Custom Form.
This is because with a native form (as you are using now), no values are written to the table until the form is submitted, and so there is no data to do any calculations with.
The only way to do this is by using a Custom Form.
This is because with a native form (as you are using now), no values are written to the table until the form is submitted, and so there is no data to do any calculations with.