Spectrophotometric fits of the X-shooter data for our sample white dwarf WDJ0902–0406. Top-left panel: SED fit between the observed spectrophotometry (black) and best-fitting model (red). The adopted atmospheric parameters are indicated. Bottom-left panel: flux residuals from the corresponding SED fit. The black line is the calculated residual, and red horizontal lines show residuals of 0 and ± 3% as a guide. The vertical blue bands indicate wavelength ranges contaminated by telluric lines. Right panel: Balmer line fits for Hβ to Hζ between the observed spectrophotometry (black) and best-fitting model (red). The line profiles are vertically offset for clarity. Analogous plots for the remaining 13 flux standard white dwarfs are shown in Appendix A.