It is more important, and much more difficult, to calibrate amplitude and phase fluctuations that result from changes in the propagation properties along the atmospheric path to the target. Unfortunately, no calibration based on observations of a phase reference source that is not in the same isoplanatic patch as the target can be guaranteed to improve the data quality. This does not mean that attempts to calibrate atmospheric fluctuations using distant reference sources are a waste of time, but you must recognize that such ``external" calibration may or may not be successful.