Computed value on multipage Webform
I can suggest you a simple way to do this. You can pass the calculated value stored in a variable to the next form function.

Like I had two step form &amp; at first step of the form user select some name from given name(checkboxes), I store them in array &amp; pass them to the next form function like --

function MYMODULE_form($names)//Use your module name instead of MYMODULE.
{
  $return_form = drupal_get_form('MYMODULE_get_form', $names);
  return drupal_render($return_form);
}
function MYMODULE_get_form($form, &amp;$form_state, $names)
{
  $form = array();
  $i = 0;
  foreach($names as $name)
  {
    $form['item']['name'.$i] = array(
      '#type' =&gt; 'markup',
      '#value' =&gt; $name,
    );
    $i++;
  }
}


Try something like this, if it solves your problem. Use simple variable if you don't have multiple values.