From the FoCal manual:
Different distances to the target can yield different AF micro adjustment results. Due to the way the autofocus system operates in a camera, a single AF micro adjustment value will almost certainly not provide perfect operation at all distances. Each result obtained at a specific distance is valid for that distance. Ideally, you should calibrate at around the “most used” distance. Canon recommend a distance of 50x the focal length (e.g. 2.5m for a 50mm lens), but this will only yield a value correct at 50x the focal length, which may not be appropriate for the lens in question (e.g. a macro lens). It is also worth noting that the AF micro adjust solution may start to move significantly as the test is run close to Minimum Focus Distance (MFD) of the lens, so it is not advisable to run the test with the camera very close to the target. It is important that the target does not move during the test (the software checks for this and alerts you if there is a problem), so the camera should be mounted on a tripod.
I do 25 - 50 x the focal length
