The key to minimizing wind resistance is to minimize boundary layer separation. This is why aero helmets are tapered. While this bit of software is useful it is designed for a rider on a bike by the looks of things so extending it to a loaded bike or a bike + trailer may be difficult. More about what model assumptions are used would be needed. At a guess I would say this is an empirical model and thus that point of source from the data my drastically change how applicable this model is. It seems like it may a useful tool for comparing different setups but don't take too much notice of it because you don't know how the model came about / what data / assumptions are used. Fluid flow is very complicated and frequently counter intuitive or at the least gives surprising results.