  For Yang-Mills theories in four dimensions, we propose to rescale the ratio
between topological susceptibility and string tension squared in a universal
way, dependent only on group factors. We apply this suggestion to $SU(N_c)$ and
$Sp(N_c)$ groups, and compare lattice measurements performed by several
independent collaborations. We show that the two sequences of (rescaled)
numerical results in these two families of groups are compatible with each
other. We hence perform a combined fit, and extrapolate to the common
large-$N_c$ limit.
