We examine Cahn theory as a quantitative method for predicting wetting behavior. Whenever a first-order wetting transition involves a state of low adsorption, it is necessary that this state be described accurately. However, we show that Cahn theory in its simplest form is not appropriate in such circumstances. By examining a discrete lattice-gas model, a modified Cahn theory is derived which provides a better description of the low adsorption regime. We apply both standard and modified Cahn theories to the adsorption of n-alkanes on water. Whereas there is a discrepancy between the temperature of the first-order wetting transition predicted by the standard Cahn theory, and the temperature of the first-order thin-thick transition found experimentally with which it is identified, this discrepancy is removed in the modified theory.