Generally, perform the AF microadjustment at the longest end of the zoom range. You can then check performance at the wide end, and if a different adjustment is required, you decide if you want to compromise. The logic behind adjusting at the long end is the longer the focal length, the shallower the depth of field. Thus, a wide angle lens is more forgiving of misfocusing than a telephoto. For the same reason, you perform the microadjustment with the lens aperture wide open (shallower DoF, focus is more critical).


I use a LensAlign Pro for my focus testing and AF microadjustment. Even if you don't use the hardware, there are some helpful tutorials and advice about AF microadjustment on the site.