Observed expansion of the outer rim of the light echo, as measured in Tylenda et al. (2005) (symbols), compared to our modelling (lines) assuming a distance to V838 Mon of 6.1 kpc. The curves were obtained taking z₀ = 2.1 pc (middle curve)  ±  10% (outer curves). The outer rim in the simulations was defined as the radius at which the echo surface brightness drops to 10% of its mean value. The light curve of V838 Mon, used in the simulations, is the same as that used to model the light echo in the F435W filter, see upper panel of Fig. 4.