  For a line bundle L on a smooth surface S, it is now known that the degree of
the Severi variety of cogenus-d curves is given by a universal polynomial in
the Chern classes of L and S if L is d-very ample. For S rational, we relax the
latter condition substantially: it suffices that three key loci be of
codimension more than d. As corollaries, we prove that the condition
conjectured by G\"ottsche suffices if S is P^2 or S is any Hirzebruch surface,
and that a similar condition suffices if S is any classical del Pezzo surface.
