Build the calculator into the program the same as Quicken. Currently, the QMEM program defaults to Windows calc.exe. However, if you have customised windows and use a third party calculator in place of the calc.exe, you are out of luck, there is no calculator function.
This enhancement has been requested in the beta testing of QMEM version 2. I also like the ability to use the calculator in the field rather that the Windows delivered calculator.