As hydrogen fuel cell vehicles become more widely adopted by consumers, the demand for refuel- ing stations increases. Most vehicles require high-pressure (either 350 or 700 bar) hydrogen, and therefore the refueling infrastructure must support these pressures. Fast running, reduced order physical models of releases from high-pressure sources are needed so that quantitative risk assess- ment can guide the safety certification of these stations. A release from a high pressure source is choked at the release point, forming the complex shock structures of an under-expanded jet before achieving a characteristic Gaussian profile for velocity, density, mass fraction, etc. downstream. Rather than using significant computational resources to resolve the shock structure, an equivalent source model can be used to quickly and accurately describe the flow in terms of velocity, diameter, and thermodynamic state after the shock structure. In this work, we present correlations for the equivalent boundary conditions of a subsonic jet as a high-pressure jet, downstream of the shock structure. Schlieren images of under-expanded jets are used to show that the geometrical structure of under-expanded jets scale with the square root of the static to ambient pressure ratio. Correla- tions for an equivalent source model are given, and these parameters are also found to scale with square root of the pressure ratio. We present our model as well as planar laser Rayleigh scattering validation data for static pressures up to 60 bar.