  In this article we discuss the possibility of using the observations by GLAST
of standard gamma sources, as the Crab Nebula, to calibrate Imaging Air
Cherenkov detectors, MAGIC in particular, and optimise their energy resolution.
We show that at around 100 GeV the absolute energy calibration uncertainty of
Cherenkov telescopes can be reduced to <10% by means of such cross-calibration
procedure.
