That sounds correct.
But I'm confused as to what you want to find out. If you want to find the maximum frequency that can be achieved in this technology, why not just make a ring oscillator? Make a chain of odd number of inverters, with the last one connected back as input to the first one. Give an initial condition on any net, and then just see the frequency at which the circuit oscillates.
If you just want to find max freq of your circuit, then keep on increasing input frequency till output fails. You can do this using .alter, sweep and other commands.