webform: redirect to modified result page
You have do some customization for this, use hook_form_alter(). 


Create a custom module.
Create a result page in that module.
Alter the webform.
Do the calculation.
Sent the data to result page and show.


Example:

function  MODULE_NAME_form_alter(&amp;$form, &amp;$form_state, $form_id) {
  if (strpos($form_id, 'webform_client_form_') === 0) {
    // webform_client_form_ (change this id with your form id)

   $form['#submit'][] = 'changeform_form_submit';

  }
}

function changeform_form_submit($form, &amp;$form_state) {
  ... your submission logic ...
}