I think QBank scores are usually poor predictors of performance. Your best bet is to take a practice test and see how you do. These are better at predicting actual performance at discrete points of time in your studying (i.e. QBanks are collection of test scores from many stages on many different subjects that might not reflect greatly on the actual distribution of questions on a real exam, etc.).
Also, I'm on my second UWorld pass and am averaging 85%. This is predicted from some sources to be above a 260. There is no way in heck that I'm even close to being at that point (i'm probably in the 220's/230's right now).