Though there is no such formula that relates OBC rank and CRL rank as it depends on various factors like total number of total candidates, number of candidates from OBC category, difficulty level of question paper. But for a rough idea I would say:

If your category rank is 500, then you CRL is somewhere between 2500 - 3500.

With such rank you have good chances to get admission in various branches in different IITs.

To have a list of all the institutes which you can get based upon your rank, you can use the following link: