Members of the MACPF/CDC superfamily of pore forming proteins have been shown to form incomplete arc-shaped assemblies as well as complete rings of proteins. As suggested previously and recently exemplified by atomic force microscopy studies on the CDC suilysin [1], both types of assemblies can form functional pores. We have built a simple model to describe the oligomeric states in which suilysin assemblies can be found, taking into account rate constants for monomer association and for binding to the membrane, as well as the initial monomer concentration. This model fits the experimental oligomeric populations with only one free parameter, and indicates that the protein assembly is arrested by depletion of monomers, before the assemblies insert into the membrane. Using atomic force microscopy on supported lipid bilayers, we have experimentally challenged the model by varying the cholesterol content of the lipid membranes and the concentration of injected suilysin, and found that steric hindrance of the arc-shaped assemblies plays a significant role in determining the final protein assemblies, in addition to the monomer depletion that is included in our model.