Pre-req gpa would just be the classes the schools lists as requirements for applying. And then theres your overall. I think that if your cumulative and your pre-req is above a 3.0 and you have a fairly good OAT score, than you should be fine for most schools, unless your shooting for a specific school.
And as for Illinois, I remember we had someone from ICO come speak at one of our Pre-Optometry meetings and she said out of 900 applicants a year they have about 300 acceptances. So 33% chance does not sound to bad... 🙂