I know how to test and adjust a prime lens. What about a zoom?

Zoom lenses present a problem when doing an AF adjustment because there is only 1 AF Adjustment in the camera for each lens. Here is a suggested practice when adjusting a zoom lens.

Let's consider a 70-200mm lens.

Start by testing and calibrating the lens at 200mm at the normal minimum suggested distance of 16.4 ft (5 m) between LensAlign and the camera. Once you have set the AF Adjustment to render proper AF performance for the 200mm end of the lens, you should then test the lens at the short end (70mm) at its normal test distance of 5.7 ft (1.74 m). If the AF performance is good at 70mm, you now have a great all purpose setting for this lens on this body. If it is off slightly, you might accept the 70mm performance or set a compromise AF adjustment that will improve both ends of the lens (but not perfectly).

If the AF Adjustment at the extremes of the lens are very different, then you will have to make a decision either to go with one or the other (depending on your shooting needs), or use them both at different times, when using the lens for different purposes. If both ends must be right, then the lens will have to be serviced by the manufacturer to bring the short and long end of the zoom into proper calibration. (Check with the service center to see if this is possible for your specific lens).