Relevant length scales for the drafting instability in the minimum mass solar nebula: the friction length (l_f), the upper scale for fast growth (λ_knee), and the scale at which viscosity dominates (λ_visc). We consider particles located at a particle scale height above the midplane, with H_p/H = 0.1 and the midplane dust-to-gas ratio is 0.1. We take the toy model parameter to be α = 0.01. Particles are assumed to be 1 cm in radius, or 1 mm in a gas depleted disc with 10 times lower gas surface density (in which case the curves remain the same, but viscous scale λ_visc,d is now the red dotted line). The instability likely does not operate in the inner (<5 AU) of the protoplanetary disc, unless particles are large or significant pre-concentration occurs.