measuring c_comp using simulation

Hi,
 
In Arpad Muranyi's "Introduction to IBIS Models" (available from IBIS
website), there was a sample Spice netlist for a simulation to obtain the
value of c_comp. When I used this simulation for my buffer, the capacitance
ranged from 2pF to 8pF, with a mean of 5pF. In the example, the range was
only between 2 to 3pF. Should the mean value for the simulated capacitance
be used? What about obtaining min and max values for c_comp?
 
I would really appreciate any feedback. I have attached the sample Spice
file for reference.
 
Thanks very much,
Betty