There's a lot of variability between Q bank scores and Step I scores, but there is some rough relationship. But it depends on a number of variables. One of the most important is simply how far in advance of the test you are doing the questions (especially if you are the typical 2nd year and are still going through large chunks of pathology). Another is how much review you have done before (and will be doing) during Q-bank.
I had a friend from last year who got the 6 month Q-bank and started off with scores in the 50's and ended up with a Step I above 250. I can think of at least 2 reasons for her low initial scores. 1.) A lot of questions were foreign to her, since she was still in the middle of 2nd year. 2.) She started doing questions before starting any kind of review, using them as a learning tool in themselves.
Conversely, I know people that got decent scores on Q-bank (high 60's, low 70's), but didn't do as well on Step I. Why? Well, part of the reason is that they used them toward the end. They had already come close to maxing out, so to speak, having had the entire 2nd year curriculum and some preliminary review, when they started q-bank.
So, when someone reports a percentage it should always be interpreted in context. If you still have 3 months before step I and have a q-bank average of 60%, that's not cause for alarm. If you have done almost all your studying for the exam and have already had the 2nd year curriculum, that's another matter.