We've been using the Iowa State (contains U of IL inputs) for years now. They have always required N cost ($$/ton) and corn price to be factored in to give best economic return values. The Illinois Fertilizer & Chemical Assn asks for farm volunteers to create on-farm plots to glean data to submit to the Iowa State calculator. There is a very specific protocol if one wants to participate. They do reimburse you for your effort.

http://cnrc.agron.iastate.edu/nRate.aspx

The following states appear in the "dropdown" choices:

Iowa

Illinois

Indiana

Wisconsin

Michigan

Minnesota

Ohio