We recently constructed phase diagrams for thin foam films stabilized with nonionic surfactants by treating disjoining pressure (TT) versus thickness (h) curves of foam films like p-V-m isotherms of real gases. However, the correlations were qualitative rather than quantitative. In the present paper, an empirically derived equation of state for a quantitative description of foam film properties is proposed. New experimental data are provided which clearly illustrate under which conditions (a) subcritical, critical, and supercritical films are formed, (b) the common black film (CBF) to Newton black film (NBF) transition is reversible, and (c) CBF and NBF coexist.