Accurate laboratory transition energies for the Ni Kα–Fe Kβ blend are urgently needed both for atomic-structure benchmarking and interpretation of astrophysical X-ray spectra observed with space telescopes. Crucially, inferred abundances of these elements, which are essential for the diagnostics of hot plasmas, depend on the treatment of those overlapping transitions. By combining a transition-edge-sensor X-ray microcalorimeter array and a superconducting electron beam ion trap, we distinguish with line widths between 3.5 eV and 5.5 eV the key transitions from Ni XX to Ni XXVII, including previously unmeasured ones, from the Kβ lines of Fe XX to Fe XXV. Collisional-radiative calculations using the Flexible Atomic Code agree well with our results. By calibrating all sensors individually and correcting for their temporal drifts we achieve an average uncertainty below 20 km/s for their transition energies, easing the interpretation of astrophysical spectra. Furthermore, our charge-state-resolved results benchmark theory for complex open-shell ions in the intermediate coupling regime, for which we compare the results from pure CI and calculations including MBPT and find a significant improvement when including MBPT.