  Scale setting for QCD with two flavours of staggered quarks is examined using
Wilson flow over a factor of four change in both the lattice spacing and the
pion mass. The statistics needed to keep the errors in the flow scale fixed is
found to increase approximately as the inverse square of the lattice spacing.
Tree level improvement of the scales t_0 and w_0 is found to be useful in most
of the range of lattice spacings we explore. The scale uncertainty due to
remaining lattice spacing effects is found to be about 3%. The ratio
w_0/\sqrt{t_0} is N_f dependent and we find its continuum limit to be 1.106 \pm
0.007 (stat) \pm 0.005 (syst) for m_\pi w_0 \simeq 0.3.
