I think the easiest way is to come back about 1mph from your normal operating speed, tell the bin driver to come in beside you and look straight ahead, dont look back, sit on a pre determined speed, and then the combine operator(usually the guy with the most experience),can fill the bin how he wants, remebering he has a 1 mph leeway when he needs to go from the back of the bin to the front. If you leave it up to the bin driver to do the filling he is contantly looking back, accidently getting offline, and has not got the view into the bin that you have from the combine. This system is by far the safest from stuffups, and the most econmical