Demonstration of a retrieval of the mass-loss rate and temperature from the helium spectrum of the generic hot-Neptune introduced in Sec. 3. a) True model metastable helium spectrum and the mock observed spectrum. The best retrieved model from the resolved line fit is also shown. b) Results of the fit to the spectrally resolved line, without using a prior. c) Results of the fit to the EW of the line. As visible from the blue contours, the fit offers no tight constraints on its own. We thus place a normal prior on the parameter space based on the model temperature self-consistency, shown in orange. The posterior distribution shown in black then yields a precise and accurate constraint.