How to calculate the power efficiency of zener diode regulator?

Is it correct to calculate it as the ratio of power delivered to load to the total power drawn from input source?

If I am correct, would the efficiency of circuit shown in the schematic below is 0.5% ?

EDIT

The maximum current zener can handle in this case is around 240mA.

Based on this,

$$ \frac{Vin - Vz}{Imax} = \frac{12-3.9}{240} = 32 Ohms $$

Am I correct?

Is efficiency Vout/Vin or Pout/Pin ?