we use yield  and/or satellite imagery maps to make our prescriptions.  We have had excellent results, haven't seen any savings on N cost, but increased yield in fields(by applying more N to the high producing areas and less to the hilltops). Where the ground is capable of producing more yield you can afford a little more N, and where it isn't capable of producing more we have decreased N without losing yields.

Zone soil sample to find out what is out there.