Here is a quick and easy test to determine if a serious micro adjustment test is required. I set these 5 AA batteries about 2 inches apart, shot them with my 1DX II (100-400 mm Canon zoom lens) at about a 45 degree angle, with the aperture wide open. I focused on the center (X) battery. From this photo, it appears as though I took the photo straight on, but I did not. Like I mentioned, it was shot at approximately a 45 degree angle. Notice that the X battery does appear to be the sharpest sharpest of the five. Those batteries to the left and the right (front and back) of the center are equally out of focus. This quick test immediately tells me that that I am not having front or back focusing issues, and it's not necessary to do an elaborate micro adjusting test.
Concerning this AA battery focusing test, how far away from the batteries should the camera be ? or is it dependent on the focal length of the lens ? and if that be the case, is there a chart depicting the distances per focal lengths ?