Dielectric polarizabilities for most of the ions in known perovskites scale with the ionic volume and the valence. These ionic dielectric polarizabilities and the ion additivity rule have been used to calculate molecular dielectric polarizabilities for perovskite substrate materials used for high-temperature superconductors. Using the ion additivity rule to predict possible low permittivity compositions seems to suggest that the constraints of the perovskite structure and stoichiometry, lattice match to high-temperature superconductors, and congruent melting required for bulk growth limit the compositions to ones unlikely to be superior to the currently available materials. The most limiting factor on the relative permittivity of the perovskites is probably the close-packed nature and lack of voids in the structure. However, in nonferroelectric perovskites, the polarizabilities derived from relative permittivity data using the Clausius-Mossotti relation are significantly less than the calculated values, with deviations that correlate with degree of cation compression. Use of cation compression to reduce the polarizability shows some promise for improving dielectric constants.