The CDF experiment is successfully collecting data from p anti-p
collisions at the Tevatron in Run II.  As the data samples are getting
larger, systematic uncertainties due to the measurement of the jet energy
scale assessed using the calorimeter simulation have become increasingly
important.  In many years of operation, the collaboration has gained
experience with GFLASH, a fast parametrization of electromagnetic and
hadronic showers used for the calorimeter simulation.  We present the
performance of the calorimeter simulation and report on recent improvements
based on a refined in situ tuning technique.  The central calorimeter
response is reproduced with a precision of 1-2%.