HFSS capacitance/impedance between two bodies

Hi, I am trying to obtain the impedance (capacitance) between two metallic objects. I have one body inside another separated by 1 mm vacuum gap. The brown body is supposed to be powered and the green one grounded. I used a cable and a lumped port to introduce the excitation.

I simulated the model for a sweep of frequencies and then I calculated the impedance of the mirror to the housing with the S-parameters: Im(𝑍𝐵) = Im(50(1 + S11/(1 − S11))), where S11 is the EM voltage wave reflection coefficient and 50 is the characteristic impedance. The result is ~-1j Ω.
However, if I calculate the reactance theoretically, it results in ~-2.6 Ω. The theoretical value was calculated considering 𝐶=(ε·𝐴)/𝑠 and 𝑍𝐵=𝑋𝐶=1𝑗2𝜋𝑓𝐶Z_B=X_C=1/j2πfC, where A=area=86834.78 mm2 and S=separation=1 mm. εr=1 since the bodies are separated with vacuum.

Is this method correct to obtain the reactance/capacitance between two bodies?
I also attach the model here