A lot of models describing phase equilibrium in critical range are used. At the same time none of them has been justified properly in terms of modern theory of critical phenomena. Based on the hypothesis of the similarity of the isobar thermal capacity on critical and near-critical isotherms near critical point and new formulation of Migdal phenomenological theory scaled equation with variables of pressure and temperature have been developed for the first time. In the framework of the approach in question the physically justified equation for critical range have been obtained for the first time. The results obtained can be applied in scaled and wide range equations of state where saturation line is a supporting curve and in calculations for thermodynamic cycles in designing refrigeration and cryogenic technique.