Calibration is needed so that Zoom knows how big (in terms of screen pixels) a widget is on the homescreen of your device, since Android doesn't provide any means of finding that out.
If you don't do the calibration, the elements in your widgets will not appear in the correct place within your widget on the home screen.
You only have to do it once, and it's pretty easy.
If you go too far, you can use the top and left arrows to go back a little.
Repeat the process with the Zoom Calibration 2x2 widget. That's a 2x2 widget so you'll need 1/4 of a screen free space on the homescreen for it. Don't worry if you can't see any shade, you can just delete the widget in that case.
Tip: you may find it easier to use the trackball to select and press the buttons.