Wirless Power Tansfer Circuit
You say that this is a simple circuit, but it is not. Both coils affect each other (mutual inductance) and the resonant frequency heavily depends on the source and load impedances.
But there exists a lot of know-how in the RFID domain which you can learn from, because RFID seeks the same design goals.
Ok, if your RX coil is much smaller than your TX coil, then you can probably ignore the RX coil first and tune the TX coil for max. magnetic field intensity which is max. coil current.
Assume your TX circuit is a resistive driver (e.g. 50 Ohms) and your TX coil circuit is a parallel resonant LC tank. Then you need to tune the tank not for resonance, but for 50 ohms impedance near resonance, which is the point of max. power transfer from source to TX coil for a given frequency.
In practice, you do not calculate this but you measure and search for max. coil current when slowly varying the capacitor value.