When a sale is ready to be paid and I select "Finish" the software displays a "Pay Method" window. This wind has the subtotal, tax, total, and balance on the right hand side, a number of payment type buttons, and, at the bottom, a "Handling" and a "Cash Back" button and a small blank field the function of which I have never investigated (but it isn't a calculator - I did try that).
If one selects "Cash Back" one gets a simple field to be filled in.
If one selects the "Cheque" button the check is assumed to be for the exact amount of purchase.
Perhaps the calculator to which you refer is not, in fact, in my version.
Apparently my problem is that we DO allow for cash back on checks.