If close to 100 Berths are available in a given class,you are DEAD CERTAIN to get a preferred Berth of your choice,say for example Side Lower.That's not to say that you won't get a SLB if only 20 - 30 berths are left but your chances are close to 100 percentile if Berth availability is more in getting a specific berth with a small request/pleading ;)) at the time of booking at PRS which i keep reiterating and i always end up getting a SL :) Good luck.