About the Shielding Caculation of Electron Accelerator

Dear Fluka experts:
    I am very fortunately to meet the FLUKA&FLAIR, and it's the greatest MC tools. But I hesitate to use it in the shielding calculation of the low energy EM (12MeV). My project (see the attachment please) is about the shielding design of an electron accelerator using in radiation processing. Can you give some advices about:
    1. According to your experience, how much difference between the FLUKA Simulation and the practical situation;
    2. In my calculation, the error is about 20% even if I increase the particle number to 1e8 (cpu time is 80 hour). I used the BIAS card to chang the importance of the region to get better error value. Are there any else methods to decrease the error while using less time?
    Thank all of you very much in advance!
    Yours faithfully
                         Xu Lijun