We've developed the Synthetic Yield generating module exactly for such cases: you know average or total yield and want to have a detailed Yield map. You enter average or total yield, units - GeoPard generate detailed yield map for you. 

Feel free to send me the field location with basic info, will do a test field for free. 

Also it works well for cases when you have part of the field covered by yield monitor, then we create the whole field yield file using that info. 

Then you can use this map to create fertilizer maps with equations or zones, calculate profit maps, etc.