There has been so many requests regarding "live computation" during node add/edit. Here's how i got mine working without breaking anything. This link pointed me to the right direction.

Create a custom module. Change all instances of MYMODULE to the name of your module

/**
* Implementation of hook_form_alter().
*/
function MYMODULE_form_alter(&$form, &$form_state, $form_id) {
  switch($form_id) {
   //here you need to know the form_id of the content type the field is attached to. you can use dpm($form_id) to get this. Let's assume the content type is called 'INVOICE';
  case 'invoice_node_form':
   //then, you need to target the computed field, in my own case, it is the VAT field. Since computed field module doesn't add #process to the form, it's safe to add yours here. Read more about hook_element_info() <a href="https://www.drupal.org/node/169815">here</a>.

    $form['field_vat']['und'][0]['value']['#process'] = array('my_custom_process');
  break;
  }
}

//now we can make our field appear as a textfield.
function my_custom_process($element, &$form_state, $complete_form){
  $element['field_vat'] = array(
    '#type' => 'textfield',
    '#size' => 8,
    '#attributes' => array(
      'readonly' => 'readonly',
      'tabindex' => -1,
    ),
  );
  return $element;
}

Then, your computed field will appear as a text field on the edit page. you can thereafter add a javascript file to autofill the textfield.

Here's a sample below.on keyup or when the value changes on the "rate" field, i want the vat field to be populated with rate * quantity. (Silly calculation)!.

(function ($) {
  Drupal.behaviors.mymodule = {
    attach: function (context, settings) {
      $('.field-name-field-rate').on('keyup change', function (){
        var rate = $('.field-name-field-rate input').val();
        var quantity = $('.field-name-field-quantity input').val();
        var vat= (quantity * rate).toFixed(2);

        $('.field-name-field-vat input').val(vat);
      });

Note: this is not the actual value saved in the db. It's only for live calculation. Your computed field settings is the one doing the actual calculation.