The time constraints in network developments inside tunnels require a characterisation of canonical tunnels in terms of propagation path losses and developing a slope model which is usually represented as a two slope model. One of the problems arisen when the canonical model tries to predict the propagation path losses inside the tunnel is to determine the distance where the slope of the propagation path losses changes. This paper analyses two of the most common approaches to determine that distance and their behaviour in different scenarios in order to clarify the radio propagation mechanisms which define the break point distance.