Atmospheric escape is believed to significantly influence the evolution of close-in exoplanets, and has been proposed to shape planetary demographics in the form of the hot Neptune desert and evaporation valley. In order to test these theories, it is crucial to obtain accurate estimates of the atmospheric mass-loss rate for evaporating exoplanets. Strong absorption during transit in the helium line at 1083 nm has recently proven a powerful direct observational tracer of atmospheric escape. A common approach to extracting mass-loss rates from these observations is to fit synthetic line profiles based on models that parametrize the planetary outflow in terms of the mass-loss rate and a constant temperature. However, such analyses often result in degeneracies between the mass-loss rate and temperature, making it difficult to narrowly constrain either parameter. We place additional theoretical constraints on the allowed temperature by simulating outflow models with the photo-ionization code Cloudy, which ensures a highly detailed NLTE treatment of the gas. Combining the two constraints allows us to find much tighter estimates for the atmospheric mass-loss rate. We demonstrate this here for the helium line, but our methods can be applied to many other spectral lines in the visible and ultraviolet wavelength range, which are inherently included in Cloudy, in order to probe different layers of the escaping atmospheres and thus get a better insight into their properties.