I need to exclude some master categories from group pricing.

It looks like it needs to be done in the
function calculate_deductions($order_total)

in ot_group_pricing.php

Cant figure out how to get the order totals for only the excluded master categories so they can be deducted along with gift vouchers in the line below.

$discount = ($orderTotal['total'] - $gift_vouchers) * $group_discount->fields['group_percentage'] / 100;

Is it possible to do? Any help greatly appreciated.