Ability to tally amounts in the $ boxes

Could we have the ability to do basic sums in the $ boxes of the bills, receipts, payments and invoice forms, rather than going outside the program to use a calculator? Such as is available in Reckon desktop program.

It would be useful for changing the GST inclusive/exclusive amounts in bills and invoices line by line. Also it is not always appropriate to add another row but rather just add amounts to present as a total.