You have to point the path to a folder inside of your map. Then you can copy the PrecisionFarming.xml from inside of the Precision Farming mod into your map and edit it from there. You also don't need the whole path, just start at your map mods base folder.
So if your map mod has a maps/config folder and you wanted to add a precisionFarming folder to that. Your path would be path="maps/config/precisionFarming". Like that.
Also, the game (or map) has to have a liquid lime fill type and spray type for it to be mapped to PF. I use the values in spray type mapping to figure out what type of spray each one is so I can tell PF how to handle it.