Calculate mutual impedance from simulation software (NEC)

Hi there,
Anyone can show me how to calculate mutual impedance of 2 dipole antennas (one dipole is driven while one is short circuit) using input impedance obtained from NEC?
I'm using this: Zmutual=sqrt[Zself*(Zself-Zinput)].

However, it didn't match with the calculation I used by Induced EMF Method.This method (EMF) matched with previous conference papers, so I assumed it is correct.

Is it something wrong with my simulation (NEC)?

Many thanks