So you will need to calibrate the camera in your Android device if:
1. You bought a wide lens adapter for your camera, something looking like one of these: 
2. Or you want to make sure that the Magic ViewFinder app has the most precise data about your built-in camera.
Calibration is a very simple process that will take 3 minutes. So do it, and it will be good for you.
1. Place your Android smartphone or tablet vertically on a flat surface, the floor or a table. The camera should be aimed horizontally, not down or up. I usually just squeeze my smartphone in between heavy books.
2. Launch your standard Camera app in the device and make sure it is producing the image in 16:9 full screen ratio, not 4:3. (Also, if your Camera app has zooming function, zoom out as wide as you can).
3. Now place 2 objects on the surface in front of the camera so that they come close to the left and right edge of the frame on the device screen respectively.
4. Measure the distances between them, and also between the lens of the built-in camera and each of the objects (you get three values). You are almost done.
5. Switch to Magic ViewFinder app, go to Menu > Calibrate device > Calibration Guide. After 2 slides with explanation about the calibration process, you will get to the page where you must enter the measured distances. The page pretty obviously tells you to do that. Having entered the distances (in centimeters or inches), press Calculate the angle. Done! You can see the angle in degrees now. Press Set.
Now Magic ViewFinder will keep this angle value until you decide to change it.