I read on Ankur Gupta's website www.ankur-gupta.com/blog that NIT first counseling goes up to 8000 AIR while second counseling goes up to 12000 AIR. From this I inferred that at whatever rank a subject finishes in the first counseling, say AIR 10000 for instance, there is a good chance that it will finally go around 50% more of that rank, ie. 15000 in this case. To be realistic you can expect a difference of 25-50% I guess...
Let the cutoff rank in First Counseling be X.
If your rank lies within:
Sure shot chance!
Almost sure shot chance!
Very slim chance!
These figures are especially valid for AIR equal or better than 10000 in first counseling. The statistics may change for ranks which are >10000 to some/great extent.
These Cutoffs are basically for AIEEE Second Central Counseling for NITs, PEC, Thapar etc...