I took it about 2 weeks prior to my exam and scored pretty close to my average. Feels a lot different than the actual test (much more straightforward and straight up recall) but I think they correct and update the scale as people continue to take it (some older threads implied that the same number wrong had changed scores on the same NBME in different years). I think I got like 12 wrong and that was a 260 and I have a feeling that when that first came out (like NBME 18 now where 17 wrong was a 260), 12 wrong was a bit higher than a 260.
TL;DR - questions are very straightforward vs actual but the scale is probably still a good predictor.
If you are just going to use a couple NBME's though, I'd stick with 15 and up.