Mid to large-size imaging atmospheric Cherenkov telescopes for gamma-ray astrophysics have a typical threshold of ~(20 – 100) GeV in the very high energy domain. In this energy range sensitive observations of the Crab Nebula can reveal the emission from the Crab pulsar at phases P1 and P2. Observations of the Crab pulsar by the MAGIC telescopes show that the P2/P1 is a monotonically increasing function of energy in some certain range of energy. In a few tens of GeV energy range MAGIC sensitivity overlaps with that of the Fermi LAT mission, which was absolutely calibrated before the launch as well as keeps monitoring it during the flight. Comparison of the P2/P1 ratio from the MAGIC and Fermi LAT Crab pulsar data allows one to cross-calibrate the two instruments. Here we suggest using the P2/P1 ratio from the Crab pulsar observations for absolutely calibrating the operational energy range of mid to large-size IACTs.