For zoom lenses, you will be advised if the focal length is not set to either the wide or telephoto end of the zoom range.
This will be an advisory indication – i.e. you can continue to run tests even though you are seeing this warning.
Calibrating at mid-zoom positions can be interesting but usually the final value is not appropriate to be programmed into the camera.
For cameras which support only a single AF Microadjustment / Fine Tune point for each lens, you should ideally calibrate at the telephoto end of the zoom range.
For those cameras which support two AF Microadjustment / Fine Tune points, you should calibrate at the full extents of the zoom range in order to get appropriate values to set in the camera.