How to calculate subtotal,tax,packingcharges and others charges in javascript and php?

I have written a functions to calculate subtotal+tax+Other Charges.

<script>

 function mkTotal() {

  stot = parseInt(document.getElementById('stot').value);

       fwd = parseInt(document.getElementById('fwd').value);

       oth1 = parseInt(document.getElementById('oth1').value);

     var tax_rate = parseInt(document.getElementById('tax0').value);

  var tax = stot * (tax_rate/100);

   document.getElementById('tax1').value = tax;



 var total = stot + tax + fwd + oth1;

        total = Math.round(total);

           document.getElementById('net').value = total;



                       }

</script>

but the problem is in this line: tax rate is fetched by AJAX respond text 'txtHint3'.

<td width="142" ><select name="tax_name" onchange="showUser3(this.value)" >

        <option  selected="selected"  disabled="disabled">Choose Tax Name</option>

        <?php

       $sql01=mysql_query("select * from tax_master order by id ASC");

        while($val01=mysql_fetch_array($sql01))

         {

         ?>

        <option> <?php print $val01['tax_name'];?> </option>

        <?php  } ?>

   </select>  </td>

   <td width="246" ><div  align="right" id="txtHint3"><strong>Choose tax name First </strong></div></td>

  <td width="160"><input type="text" name="tax_amt" value="" style="text-align:right" id="tax1" readonly="true"  /></td>



<tr >

This is the pic of my work


I want it to be calculated as I choose tax name from the drop down list.
See
1. In 1st row 'Gtotal of sale enries' is the total calculation of sales.
2. In 2nd row Drop down list of tax names.
3. next box is of tax rate fetched by Ajax.
4. The last box in 2nd row is for calculation of tax amount(should contain 0.00 if no tax is selected).
5. 3rd Row is for adding packing & forwarding Charges(should contain 0.00 if no value is available).
6. 4rth Row is for other Charges(should contain 0.00 if no value is available).
7. The last Row is for Gross Total.

I Hope you understand the problem and can help me.