Here we encourage use of a single assumption that ∆H vap / (R∆Z vap ) is constant. This is motivated by use of NIST reference data and MATLAB code (or MS Excel template) accompanying the electronic version of this manuscript. We have shown that this is superior to the conventional approach of assuming first (1) that change in compressibility upon vaporization (∆Z vap ) is approximately 1, or equivalently that the vapor phase may be treated as an ideal gas where the molar volume of the vapor is much greater than that of the liquid, which may be assumed negligible. And second , that the enthalpy of vaporization (∆H vap ) is constant. While the conventional approach is able to explain linearity, it can be misleading as both ∆H vap and ∆Z vap are known to go to zero at the critical point.