Entering a withdrawal it would be useful to have a tax option that added the gst and pst from the net amount entered

i have payments that i make that have more than one line on the receipt showing the net amount and some have gst only and some have gst and pst. For the ones with both taxes i have to calulate the total and then put both taxes in amount, it would be easier to put the net amount and have an option which says add both taxes