There's nothing like a formula for the cutoff . It all depends on the candidates and the toughness of exam and thus it varies every year and thus you cant calculate that how much candidates are eligible to score more or less using a formula. Just go through or college predictor and get to know about approximate college you are eligible for. And yes, the college predictor doesnt works on  formulae but just takes a few last year's cutoffs and gives the approx variation for present.