Deriving an Empirical Formula to Predict Resonant Frequency of a Microstrip Antenna

Hi all,

I need help to derive the formula for my designed antenna. This formula can be used to predict the resonant frequency. The patch is as shown in the picture. Substrate is FR4 of 1.6mm thick with ground plate on the other side. It is a coaxial fed antenna.

Any ideas on how I should start? If you do have experience in this, please do share.

Thanks in advanced