Calculate fields Via Visual Editor

Hi Support,
I have placed the code snippet in Visual Editor on the invoices_add.php
<script>

{literal}

var x = document.editform.value_net;

x.onchange = x.onkeyup = function() {

var net = document.editform.value_net.value;

var vat = document.editform.value_vat.value;

var gross = document.editform.value_gross.value;

document.editform.value_vat.value = net * 0.175;

document.editform.value_gross.value = net + vat;

document.forms.editform.value_gross.disabled=true;

document.forms.editform.value_vat.disabled=true;

} {/literal} </script>
When executed i get the following page:

When i enter a value for document.editform.value_vat.value = net * 0.175;)
However, the gross is does not calculate correctly.
gross = net + vat
Please Help
Lisa

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