Example calculation of critical projectile-to-target mass ratio (in blue) and the classical damping rate given by Eq. (5) for 1 mm grains as functions of radius in a wide disc around a solar-mass star, with particle $Q_{\text{D}}^{*}$ given by model 3 in Table 1, an average eccentricity of 〈e〉 = 0.01, particles ranging from 1 µm to 100 m in size with a power-law size distribution exponent q = 3.5, and a disc mass of 0.1 M_⊕. This example illustrates how, in a wide disc, collisional damping may be a non-monotonous function of radius: inefficient at short radii due to low projectile-to-target ratio and slow at large radii due to low collisional rate, with a ‘sweet spot’ in the middle where damping is both efficient and fast enough to be observed assuming, for example, a system age of 100 Myr. See Sect. 5.1.