We developed a technique and selected the optimal parameters for solving the inverse problem of determining the total content of R-11 freon (CCl3F) from ground-based spectrometric measurements of solar radiation at the NDACC St. Petersburg station using a IFS125HR Fourier interferometer and SFIT4 software. The main features of the technique: the used spectral interval is 830-860 cm -1, the previously measured spectral sensitivity function of the device is taken into account, and the continuum absorption of radiation by water vapor and attenuation of radiation by amorphous ice deposited on the receiver are also taken into account. Moreover, when solving the inverse problem to compensate the variability of the thickness of the ice film, the shape of the baseline of the spectrum is corrected by a polynomial of the second degree and the vertical distribution of water vapor is specified. Using the technique, the total content of R-11 was determined in the period 2009-2019, and an estimation of measurement errors was obtained: the average systematic error was 7.4% and the random error was 2.9% for the entire observation period. The assessment of the long-term total content trend is -0.29 ± 0.07%/year, and the mean molar fraction trend is -0.31 ± 0.07%/year.