Stereo audio signal is often modeled as a mixture of instantaneously mixed primary components and uncorrelated ambience components. This paper focuses on the estimation of the primary-to-ambience energy ratio, PAR. This measure is useful for signal decomposition in stereo and multichannel audio coding, format conversion, and spatial audio enhancement. The conventional approaches for the estimation of the ratio are based on the ratio of eigenvalues which requires equal energies of the ambience signals. This often leads to an inaccurate estimate of PAR. An alternative measure is proposed which reduces those estimation errors but requires a priori information about the primary component signal. The performance of the method is demonstrated with synthetic signals and a large collection of stereo audio data.