  A leading order (LO) analysis is presented that demonstrates that key top
backgrounds to H -> W^+W^- -> l^\pm l^\mp \sla{p}_T decays in weak boson fusion
(WBF) and gluon fusion (GF) at the CERN Large Hadron Collider can be
extrapolated from experimental data with an accuracy of order 5% to 10%. If LO
scale variation is accepted as proxy for the theoretical error, parton level
results indicate that the tt~j background to the H -> WW search in WBF can be
determined with a theoretical error of about 5%, while the tt~ background to
the H -> WW search in GF can be determined with a theoretical error of better
than 1%. Uncertainties in the parton distribution functions contribute an
estimated 3% to 10% to the total error.
