Monolayer films at high surface pressures are well-ordered, and due to their 2D crystallinity they can act as sites of preferential nucleation. Recently it has been demonstrated that films at lower surface pressures can also be preferential nucleators, despite their decreased ordering. In this study we show that identifying the factors which can reduce the interfacial tension is vital to understanding the varied mechanisms through which nucleation beneath monolayer films can occur. In particular, the preferential nucleation that is observed at low to medium surface pressures is explained by the compressible film rearranging so as to minimize the lattice mismatch with the nucleating crystal face, thereby reducing the interfacial tension of the system. We introduce a more quantitative approach in this paper to gain a greater insight into this phenomenon. The nucleating species must be adsorbed upon the film as a prerequisite for nucleation. We demonstrate that the extent of this adsorption may be modeled using the appropriate Pi-A curves and an equation derived from the Gibbs and Langmuir adsorption isotherms. This approach also allows values for the adsorption energy and interfacial tension for 100% adsorption to be estimated.