I think, the question was about momentary estimation of the capacitance before the simulation. HFSS is a simulator, not a calculator and it can't give you the capacitance at the output. Just some matrices including simulated semi-infinite ports. You' get a hell of work to extract the actual impedance then and to guess the capacitance from the equivalent model and some approximate formulas.
Simple formulas acquired with conformal mapping work better but still the approximation is very rough. I didn't see any precise formula or formula covering general "sandwitch" case with metalization. There's always no sreen/ground and second layer is ε=1 that simplifies things, but doesn't take you anywhere. So, you either get simple microstrip case or simple coplanar case and not more :-(
So, if you have a guess, a software, a webpage calculator or working formulas for sandwich, please, tell us.