The Tri-State Fertilizer recommendations are a collaborative effort among agronomists and like-minded experts from Indiana, Ohio, and Michigan. This spreadsheet implementation using Indiana-centric equations does the math for you. If you are new to this style of Excel spreadsheet, there is a brief tutorial video explaining how to use the tool.
This Excel workbook also includes a side-by-side comparison tool to help you evaluate cost differences of different combinations of fertilizer to deliver nitrogen, potassium, phosphorus, and sulfur. Be sure to update the prices table to make sure your analysis is appropriate.
The latest version of the Tri-State recommendations are here in a 56 page document: