As JWST and other next-generation observatories begin to operate, follow-up observations of planets discovered by missions like Kepler and TESS will enable exceptional characterization of exoplanetary atmospheres. Understanding the stellar environments these planets reside in is critical to these efforts, with the UV regime of the host star’s spectrum driving important non-equilibrium chemistry — particularly photochemistry. Before JWST begins observing, we must understand how to best leverage active UV-capable observatories such as HST to constrain these host star spectra. Using the MUSCLES Treasury Survey, we quantify how using reconstructed proxy UV spectra based on Ca II H & K optical lines (mimicking situations in which a planet’s host star has not been observed in the UV) impacts photochemical modeling of exoplanetary atmospheres. We use the Atmos 1-D coupled photochemical and climate model to simulate habitable zone planets in orbit around each of the stars in the MUSCLES catalog to determine the planets’ atmospheric thermal profiles and chemical compositions using both observed (actual) and reconstructed (proxy) versions of the host star spectrum. We furthermore generate the resulting transmission spectrum in each case, to quantify the impact of the stellar UV reconstruction on the observable properties of the atmosphere.