Hey Ananda

To get admission in JJ college of architecture the marks is calculated as your boards percentage + half of your nata score. For general category it is around 180 (according to past few years) however this year it may change as both cbse as well as isc board has given higher than usual marks to the candidates.

As far as the rank in NATA goes you need to have  a rank below 500 to be a contender for admission.

Hope this helped. All the best.