This has been discussed on ValueMD before, but I'll tell you what I've heard, anyway.
There's a formula that some use. It varies, but I like this specific one the most:
(your consistant avg x 2.33) + 83 = estimated Step 1 score
I know of two people's real USMLE Step 1 scores that I can comment on using this formula.
One of them had a 69 average on UWorld, the other had a 74 average.
Plugged into the formula, those averages give you: 243.77 and 255.42.
In real life, those two folks got a 243 and a 251 respectively.
So if a test taker stays consistent, and doesn't freeze up on exam day. One might say that this formula is pretty indicative.
However, some have said that its flaw is that the lower the UWorld score, the less accuracy this formula has...
Basically...this formula states that a 50% UWorld average is equivalent to a ~200 on the Step. However, when these same students take an NBME, their Step1 prediction on that exam is much lower. Thus, the consensus has been that that formula only works for high UWorld averages, and that one should not attempt to take the Step if he/she is sporting a UWorld average below...let's say...55-60%.