Development: Adding Invoice Subtotal (excl tax)

Over here in NZ we often have our invoices show eg the following:

Subtotal: $40 (excl Tax)
GST : $5 (Our tax - 12.5%)

Invoice Total: $45 (total incl Tax)

The subtotal variable didn't seem to be there so I've added the following code to include/sql_queries.php on line 340.

#invoice sub total excl tax
$sql ="SELECT SUM(gross_total) AS sub_total, SUM(total) AS total FROM {$tb_prefix}invoice_items WHERE invoice_id =$id";
$query = mysqlQuery($sql) or die(mysql_error());
$result = mysql_fetch_array($query);
//$invoice['total'] = number_format($result['total'],2);
$invoice['sub_total'] = number_format($result['sub_total'],2);

Hope I have done that ok, just copied the tax calculation and changed a bit.

That part was just into the include/sql_queries.php file to make the variable available to work with when creating a new template.

I'm just working on making a clean template now, but needed to do the above first to the sql_queries file.

Sounds good. Comments welcome, I'm new to this :)