Operating a high-energy accelerator like the Large Hadron Collider (LHC) requires a state-of-the-art monitoring system for radiation protection. In the vicinity of the accelerator as well as in the accessible areas behind thick shielding, a unique mixed radiation environment is encountered that consists of different particle types with energies ranging from fractions of electron volt up to several giga-electron-volts. Consequently, the correct assessment of ambient dose equivalent poses a challenging task and requires appropriate field-specific calibration methods, in particular as no adequate calibration sources exist. This circumstance motivated the development of a more accurate field calibration method for the LHC, based on benchmarked FLUKA Monte Carlo simulations. The method of obtaining such field calibration coefficients for IG5 high-pressure ionization chambers is exemplified in a case study for the LHCb experiment. Comparing these factors to calibration source-based values shows over- or underestimation of the actual dose by the source-based coefficient, depending on the location of the monitor.