The dependence of efficiency of a gas-flow proportional counter with respect to the source thickness and α-particle energy was evaluated using the code SRIM. Results for the mass-efficiency curves were evaluated for 209Po and 241Am in a substrate of NaNO3. The results revealed good agreement with a linear and a hyperbolic dependence for low and high thickness sources, respectively. However, some deviations were found for very thin thicknesses, which can be explained by taking into account scattering effects. A function is proposed for predicting the dependence of efficiency with α-particle energy in the range from 4 to 8 MeV.