For a physical adsorption system, the distances of adsorbates from the surface of a substrate can vary significantly, depending on particle loading and interatomic interactions. Although the total adsorption energy is quantified easily, the normalized, per-particle adsorption energies are more ambiguous if some of these particles are far away from the surface and are interacting only weakly with the substrate. A simple analytical procedure is proposed to characterize the distance dependence of the physisorption strength and effective adsorption capacity. As an example, the method is utilized to describe H-2 physisorption in a finite bundle of single-walled carbon nanotubes.