We propose an approach to selecting the optimal parameters for solving the inverse problem for determining the total freon content (TC)from ground-based spectrometric measurements of solar radiation. The approach was developed for measurements at the St. Petersburg NDACC station using a Bruker FS125HR Fourier-transform interferometer and implemented as applied to measurements of the total content of the hydrochlorofluorocarbon R-22 (HCF2Cl). Based on the optimal set of parameters obtained, we retrieved the total R-22 content above the St. Petersburg station in the period 2009-2018 and obtained estimates of the measurement uncertainties: average systematic uncertainty 4.8%, random uncertainty 3.7% over the entire observational period. A preliminary estimate of the trend is 2.64 +/- 0.22% per year.