How to set lens axis perfectly vertical
You can use one of these methods, or develop your own.
1. If your camera has electronic level, just use it.
2. You can also use traditional level. Every camera (or at least every one that I know) should have LCD surface parallel to image sensor, so you can put level on LCD.
3. Method a la MacGyver - use anything you have around. Attach something small and heavy to one end of a cord or string: a ring, nut or plasticine ball. The other end of the string attach somewhere near the camera. The nut or ball should hang freely over the floor. Measure the distance between string and centre of the lens.
Take a sheet of paper and draw two crosses in the same distance between them, as previously measured. Lay this paper on the floor. One cross should be under item tied to a string, second under the lens. Correct camera position, to see this cross exactly in the centre of image. To get better accuracy, use image magnification in LV mode.