In ab initio modeling of doped semiconductors, estimation of defect formation energies involving substitutional sites of ternary compounds is ambiguous due to an approximate treatment of chemical potential of the substituted atoms. We propose a model of assigning fractions of the formation energy to individual atoms of a ternary semiconductor and test it on InGaAs. The accuracy of this approximation is on the order of 0.1 eV/atom and is expected to be sufficient for many practical purposes.