Till the NTA does not release the question papers, it is very much difficult to predict the percentile (as it will depend upon the difficulty of the question paper). Also, after the question papers are out by NTA, you will not get the exact percentile until the NTA releases the final result.

However, on an average, if someone scores 200 out of 300 in JEE Main, (s)he can expect a percentile around 97-99. If the paper was too easy or too difficult for his/her slot, then it will fluctuate with a very large gap. It can go as low as 89 to 90 or as high as 99.5 to 100.