Simply extend each internal hedge into a dollar amount. At the end of a month, you plug in a closing entry vs. the closing price for the net position. The challenge is to get internal hedges entered into the computer correctly. Errors will be made, of course, but they're easy to catch by checking and reconciling with the locations daily. A fax check is better than a phone call.
Errors are more difficult to find once they've aged, so perform checks daily.
This must be managed, of course, but a certain amount of conflict can be good. Consider overnight price risk, for example. Location managers want to buy unchanged to originate bushels, while the central hedger wants to be quick to take protection. Giving each of these conflicting objectives its own sponsor may lead to better decision making. This is possible only if risks are taken that have nothing to do with cash grain trading.