  The evaluation of the numerically dominant pseudoscalar-pole contribution to
hadronic light-by-light scattering in the muon g-2 involves the
pseudoscalar-photon transition form factor F_{P gamma^* gamma^*}(-Q_1^2,
-Q_2^2) with P = pi^0, eta, eta^\prime and, in general, two off-shell photons
with spacelike momenta Q_{1,2}^2. We determine which regions of photon momenta
give the main contribution for hadronic light-by-light scattering. Furthermore,
we analyze how the precision of future measurements of the single- and
double-virtual form factor impacts the precision of a data-driven estimate of
this contribution to hadronic light-by-light scattering.
