  We derive exact expressions for the excess number of clusters b and the
excess cumulants b_n of a related quantity at the 2-D percolation point.
High-accuracy computer simulations are in accord with our predictions. b is a
finite-size correction to the Temperley-Lieb or Baxter-Temperley-Ashley formula
for the number of clusters per site n_c in the infinite system limit; the bn
correct bulk cumulants. b and b_n are universal, and thus depend only on the
system's shape. Higher-order corrections show no apparent dependence on
fractional powers of the system size.
