I have created a fixed price coupon for $20 but when it is used it calculates the discount of $22.

I am from Australia so have 10% GST so it seems to be adding this to the coupon.

I have commented out the code in includes/modules/order_total/ot_gv.php that is mentioned in the known bug fix thread but did not fix the problem.

Any hints on what could be causing this?
Thanks