  We report a measurement of the ratio of the top-antitop to Z/gamma*
production cross sections in sqrt(s) = 1.96 TeV proton-antiproton collisions
using data corresponding to an integrated luminosity of up to 4.6 fb-1,
collected by the CDF II detector. The top-antitop cross section ratio is
measured using two complementary methods, a b-jet tagging measurement and a
topological approach. By multiplying the ratios by the well-known theoretical
Z/gamma*->ll cross section, the extracted top-antitop cross sections are
effectively insensitive to the uncertainty on luminosity. A best linear
unbiased estimate is used to combine both measurements with the result
sigma_(top-antitop) = 7.70 +/- 0.52 pb, for a top-quark mass of 172.5 GeV/c^2.
