Jawaduddin,
While I cannot comment on your expected value of 80% extraction but I can point out your boundary conditions are set incorrectly.You have your +/- Z boundary conditions set as APML which introduces loss, this should be changed to PBC (Periodic boundary conditions). It only marginally improves your extraction to 56% instead of 50%.
Can you provide your reasoning for your expected value? A paper or reference perhaps?
Scott