A coupled thermodynamic/kinetic CALPHAD type modeling of the glass transition for a glass forming unary substance is proposed. In this quantitative modeling, the vibrational contributions to the thermodynamic functions of the crystal and liquid/glass phases are classically modeled using weighed sums of Einstein functions while the configurational contributions to the liquid/glass phase functions are described using a single internal variable within the frame of the ideal two-state model. The freezing kinetics of this internal variable on cooling is calculated with an Adam-Gibbs logarithmic relaxation law. The model is applied to the boron oxide B2O3 and, after a numerical optimization of the parameters, is shown to well represent the hysteresis loop of the heat capacity detected by DSC in the glass transition range during cooling/reheating cycles at various rates. The model also allows to calculate the fictive temperature and residual or zero-point entropy of the glass.