Stellar high-energy radiation shapes the evolution of exoplanetary atmospheres, particularly around cool K- and M-dwarf hosts, by driving atmospheric escape and altering compositions relative to host-star chemical abundances. The dominant driver, extreme ultraviolet (EUV) radiation, is largely unobservable yet controls metastable helium, which is the main tracer of atmospheric escape in near-infrared transit observations. This introduces major systematic uncertainties in atmospheric retrievals from current and upcoming facilities (e.g., JWST).To overcome this limitation, we use X-ray observations from XMM-Newton to constrain coronal plasma at temperatures of ≈1–10 MK, closely linked to EUV-emitting regions. This coronal temperature structure can be characterized through the emission measure distribution. We show that commonly used low-resolution fitting approaches with variable coronal elemental abundances can introduce significant systematic biases, artificially adjusting abundances to compensate for unresolved temperature structure. We therefore test simplified fitting approaches, with fewer degrees of freedom, that constrain the temperature distribution even in lower-resolution data for more distant stars.This temperature structure provides essential input for spectral synthesis tools that reconstruct the EUV spectral range that governs metastable helium populations, and enables atmospheric escape models to assess the long-term stability of planetary atmospheres. This approach extends EUV reconstruction to a much broader stellar sample, overcoming a key observational bottleneck and strengthening the link between stellar high-energy radiation and observed atmospheric escape.