If you look at the resources in the Windows Accessory calc.exe then you will see that the accelerators table has many entries essentially as John describes. You will also see that the only controls that are tab stops are the data entry edit control, radio buttons and checkboxes. The push buttons are not tab stops. However if I click a radio button then the tab key seems to not work and not be necessary, so there seems to be something that makes the tab key unnecessary for returning focus back to the data entry box.

Also, if you looked at the VC sample programs, then you probably found MFCCALC, which is probably all you need.