How to calculate the power efficiency of zener diode regulator?

[![enter image description here][1]][1]

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% ?




  [1]: https://i.sstatic.net/NmHmz.png


**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?