What distance should I use for auto focus calibration?
Target distance can be a little confusing at first, essentially the distance at which you calibrate can make a difference to the calibration result. It's important to use a distance that will enable the lens to work well for a variety of distances not just the one picked for calibration.
Easiest way to determine a good target distance is the Target Distance Tool.
You should ignore 'crop factor', a camera with smaller sensor size does not change the distance used for calibration. Use the focal length listed on the lens itself as the focal length for the target distance tool.
Measuring the target distance does not need to be exact, the Target Distance Tool provides a 'minimum recommended distance' not an absolute distance for calibration. If users do wish to measure exactly the measurement should be from the camera sensor (usually marked on top of the camera).
The document What’s the correct distance to run tests at? contains further useful information and explains why target distance is important and why the distance used does not need to be precisely measured in most cases.
With zoom lenses treat each end of the zoom range as effectively different prime lenses, it's worth reading the FAQ entry How do you calibrate zoom lenses? for more details.