For a number of snapshots of the simulation, the density of a Plummer sphere at the shell position (assumed to be the ISM density) versus the moment at which in the numerical integration a shell velocity of 4 km s⁻¹ and a radius of 25 pc is reached. Different symbols denote different stellar wind efficiencies η. Shaded regions denote the envelope allowed for all efficiencies plotted and a 10% error in the observed velocity and radius. The top panel shows the results obtained using the Lancaster model, and the bottom panel shows the results from the Weaver model.