The time constraints in network developments insidetunnels require a characterisation of canonical tunnels interms of propagation path losses and developing a slope modelwhich is usually represented as a two slope model. One of theproblems arisen when the canonical model tries to predict thepropagation path losses inside the tunnel is to determine thedistance where the slope of the propagation path losses changes.This paper analyses two of the most common approaches todetermine that distance and their behaviour in different scenariosin order to clarify the radio propagation mechanisms whichdefine the break point distance.