The ITER-oriented JET research program brings new requirements for the low-Z impurity monitoring, in particular for Be - the future main wall component of ITER. Monitoring based on Bragg spectroscopy requires an absolute sensitivity calibration which is challenging for large tokamaks. This paper describes the calibration method used for the BeIV channel (75.9A) of the Bragg rotor spectrometer deployed on JET. The calibration technique presented here relies on multi-order reflectivity calculations and measurements of continuum radiation emitted from helium plasmas. These offer excellent conditions for the absolute photon flux calibration due to their low level of impurities.