add_action('cred_submit_complete', 'tssupp_calc_parent_child_distance',10,2);
function tssupp_calc_parent_child_distance($post_id, $form_data)
{
  $forms = array(123, 456);
  $relationship_slug = 'book-chapter';
  $parent_address_slug = 'book-address-1';
  $child_address_slug = 'chapter-address-1';
  $child_distance_slug = 'chapter-single-line-1';
  $unit = 'km';
  $decimals = 2;
 
  // if a specific form
  if ( in_array( $form_data['id'], $forms ) )
  {
    $related_post = toolset_get_related_post( $post_id, $relationship_slug, 'parent' );
    if( $related_post ) {
      $child_location = get_post_meta( $post_id, 'wpcf-'.$child_address_slug, true );
      $parent_location = get_post_meta( $related_post, 'wpcf-'.$parent_address_slug, true );
      if( $child_location && $parent_location ) {
        $distance = do_shortcode("[toolset-maps-distance-value location='" . $child_location . "' postmeta='wpcf-". $parent_address_slug . "' postmeta_id='" . $related_post . "' unit='" . $unit . "' decimals='" . $decimals . "']");
        update_post_meta( $post_id, 'wpcf-'.$child_distance_slug, $distance );
      }
    }
  }
}