One proposal would be to use a photodiode. In this paper: Review of Scientific Instruments 85, 075119 (2014); https://doi.org/10.1063/1.4890665 we did the following: "The flux from the X-ray source and the transmission of the high-pressure window were determined using a 10 mm × 10 mm photodiode (AXUV100, Optodiode, USA) and a current amplifier (DLPCA-200, Femto, Germany)."